I think some confusions regarding innateness of gender here are due to the (two or) three different meanings of gender and a mix-up between "biological" and "genetic".
Let's start with these four words:
(physical) sex: female ------- intersex --------- male
(psychological) gender (identity): female -------- androygnous, agender, multigender and other genderqueer identities ---------- male
gender presentation/expression: feminine ---------- androgynous, genderneutral ------------ masculine
(social) gender (roles): stereotypically female --------------- both or neither-nor ---------------- stereotypically male
(Line 3 gender expression could be considered to be part of line 4 gender role.)
(Completely independent: sexual orientation: hetero-, homo-, bi-/pan-, asexual.)
[Note: Some insist on putting "girl/woman -------- boy/man" in the second line. I do not see female and male as the terms for physical sex and man/woman as the terms for gender identity ... this just leads to terminology like "female man" "male woman" ... if this works for some trans people, fine, but most don't seem to like this.]
So, let's start with the physical sex. We have a lot of sexual attributes and for 99.5% of the population these are aligned in one of the categories male and female (at least Wikipedia mentions that 1 in 200 babies has some sexual ambiguity ... other places mention 1 in 5000, maybe those only mean children with amgibuous genitalia).
female: clitoris, labia, vagina, uterus, two ovaries producing estrogen and progestogen and eggs, breasts, no beard, less body hair, a higher voice, female facial features, narrow shoulders, wide hips, period, average lower height, typically XX chromosomes
male: penis, prostata, two testes producing testosterone and sperm, beard, more body hair, lower voice, wide shoulders, narrow hip, average higher height, typically XY chromosomes
(You already see that not all of these apply to all non-intersexual people ... some men have close to no facial|body hair while some women do, little girls and old women and some women in between don't get a period and so on.)
intersex: ambiguous genitals (what's the difference between an enlarged clitoris and partially grown-together labia and a micropenis that is partially open at the lower side? None.) and/or a vagina that ends in a "dead end" with no uterus and/or gonads that may contain both ovary cells and testis cells and/or some body cells may contain XX, others XY, others only one X chromosome and/or all the body cells may be resistant to testosterone (I think the XY women who can get pregnant have this, but not sure) and/or some genes that belong on the Y chromosome have translocated to an X chromosome and/or a number of other possibilities to be intersexual.
So is sex genetic? Most of the time. But sometimes not. Most ways to be intersexual are not genetic. Some ways to be intersexual are genetic, but still a person with a Y chromosome may end up with a - sometimes even fully functioning - uterus and other female sexual organs.
Is sex biological? Definitely. The embryo is exposed to sex hormones from the mother and to sex hormones produced by its own developing gonads. If this happens at the standard times, all is well. If there are too much or too little hormons from the mother or the embryo's gonad's develop to early or too late or produce too little or too much of their sex hormons, this can lead to certain types of intersexuality like ambiguous genitals.
Next thing up: (psychological) gender (identity) - what is meant by "gender" in most discussions of transgender/cisgender/transsexuality/cissexuality/genderqueerness. How we think of ourselves, what we feel what we are.
(Mainly) Two possible definitions of transgender:
The (psychological) gender (identity) is not the same as the (physical, anatomical, genital) sex.
The (psychological) gender (identity) is not the same as the (legal, social) gender (coercively) assigned at birth.
(For some transgender people (it seems mostly for trans men) only the second definition is acceptable, for others both are equally okay.)
As the vast majority of children are assigned a (legal, social) gender that matches their (physical, anatomical, genital) sex, this ends up referring to exactly the same people most of the time.
For some intersex people it means they could be cis by one definition and trans by the other: If a child has a Y chromosome, a micropenis, testes at birth, the micropenis is declared an enlarged clitoris and surgically made smaller (loss of sensation and sexual pleasure in later life!), a neovagina is created (which needs to be dilated - pain, experiences similar to sexual abuse) and the child is raised as a girl, but later in life identifies as male and starts taking testosterone, gets SRS (as far as still possible after these mutilations), wants to be called he, takes a male name - is this person cis or trans? It's not a particularly important question, as these people generally identify as "intersex(ual)" and don't care for being grouped as cis or trans.
So, is (psychological) gender (identity) genetical? Similar to sex: Often, yes. Sometimes, no.
Is it biological? Probably. Most likely also caused by the hormone exposures before birth, but at a different time during pregnancy, when that part of the brain is developing.
Sexual orientation is also - most likely - caused by exposure to the same hormones, at another time, with homo-, bi-/pan- and asexuality being the atypical cases because of atypical hormone exposure.
So now: gender presentation or gender expression. What clothes we wear, how we have our hair cut, how we paint our faces and nails - or not -, what style of handbag we wear - or not -, whether we pierce our ears and what we dangle from them and other jewelry and so on.
Is this genetic, biological or in some other way innate? Doesn't look like it, considering e.g. pink was a boy color and blue a girl color in the US in the 19th and early 20th century.
It's certainly influenced by a lot of things: our (true, inner) gender identity and how aware we are of it, the social gender we were assigned, our fashion preferences and how important it is to us to wear something that is comfortable, how important it is to us to fit in - and what group exactly it is we want to fit in with, how important it is to us to fulfill requests to wear something / have a certain haircut etc. from parents and other people around us and so on.
Last: social gender, gender role - in a society that has different gender roles, but is free enough that we could at least theoretically ignore some or even all aspects of them.
I would count on a similar combination of factors as for the gender expression: our gender identity is certainly relevant - our genitals probably aren't. Which gender we were raised in certainly has some influence even for trans people. Then again: How important do we feel it is to fit in, who do we want to fit in with, how important is it to match the expectations of people of authority like parents or teachers, how open are we to peer pressure - and how strongly do we feel about a particular aspect of a role. A man might feel so strongly he wants to become one of the very few primary school, kindergarten or preschool teachers that even though he has a strong desire to fit in and adhere to the male gender role that he might do it anyway. Or not.
Are gender roles genetic, biological, innate? I think, as mentioned above, the gender identity is and the wish to fit in (conformity) / openness to peer pressure as well as the wish to do what authorities (parents, teachers) ask may partially be. But the actual societal roles and expectations by gender are largely not. Testosterone does not even make people aggressive
I think differently strong desires to fit in might also be part of the reason why some trans people figure out early or easily they are trans and others have to think about it for a long time (with those with a stronger desire to fit in with their own gender maybe noticing more easily that they were forced into the wrong one). Another might be that those who find out later or find it harder to find out what gender they are might be further away from the extreme ends of the spectrum and closer to an androgynous, agender, multigender or fluid gender identity.
Gender in the sense of 3 and 4 might go away: Maybe people of both/all genders might present in the same way at some point in the future. (A first step would be to finally let men and boys wear skirts and dresses if they wish so.) Maybe there will be no different societal expectations to men and women and people of other gender identities.
Gender in the sense of 2 would still be there. "My parents tell me I am a boy|girl because I have boy|girl parts. They do not tell me to dress in a certain way and neither does society. They do not tell me to behave in a certain way and neither does society. But it's still wrong."
Maybe only transsexual people (= transgender people who also wish changes to their physical sex, by hormones or surgery or both) will notice that they are transgender at that point of time in the far-distant future, maybe not, it's hard to say and we are not going to get there in our lifetimes.
So back to Storm.
If ze is cisgender, ze will figure this out. There is no coinflip. There is no accidentally identifying as the gender opposite to one's sex because parents didn't force you to wear pink or blue or dresses or pants or play with dolls or toy guns and society didn't subtly or not-so-subtly pressure you into this. No harm done.
If ze happens to be transgender, ze will be the happiest trans kid in the world.
Like zir brothers, ze may choose clothes not typical for zir gender. Maybe only as a kid. Maybe as an adult, too. If ze suffers from it, ze will stop doing it. Problem solved.
The child is free to pick and choose activities at hir liking. No parental, grandparental, unclish/auntish, teacherish intentional or accidental (subconcious) offering of only one set of activities, or intentionally or accidentally (subconsciously) discouraging certain activities. Did you know little boys under the age of 5 smile as often as girls at the same age, but boys over the age of 5 smile more rarely?