You know what? Let's give you the benefit of a doubt go through what you do in this post.
An adult woman, whom you have no actual relationship with or place of authority over because she is your former stepdaughter, joins the military.
You have doubts about the legitimacy of her admittance and ask her - that is a +1 point, good first step.
You are unsatisfied with her answer, despite the fact you have no evidence or reason to believe she's lying about no longer experiencing those illnesses.
And you decided that you're dissatisfaction somehow overrides the decisions of her own life and her autonomy, despite being an adult woman with whom you share no relationship. That's a -1.
You do not speak to her actual parent, or some other mutual respected acquaintance who might be able to shed light on the matter. You don't wash your hands of the matter because, again, this is an adult woman who you don't have a relationship with and you've already said your piece. Instead, you escalate as far up the chain as you can go as a first resort instead of a last resort.
About the decisions of an adult woman of whom you have no relationship, who has already heard your concerns and has made their own decision.
And the reason you don't see anything wrong is because you have issues with the military and don't think its 'normal' - aka, that the fact she wanted to do this at all is something to judge and which somehow invalidates her right to make her own decisions.
like a normal girl her age
YTA