Maybe she doesn't want to have drama with a man who doesn't accept her. Maybe she has money you don't know about and maybe it's her business.
Why is my trans sister trying to transitions without telling my parents when my dad doesn't accept her and she hasn't earned one cent on her own? Why can't she wait until she has her own income?
If she is a minor .. She will need his approval for any medical treatments. If she is not, but depends on him financially .. I imagine it won't take long for him to notice she is taking steps and figure out what she is using that money for. That might cause a major delay in future monetary endorsement for him. It's just a matter of time.
I don't know. Ask her.
Why do you think it is your business?
If you think you are in the wrong body, I suspect that would be the primary issue on your mind---and pretty much everything else is a very distant 10th in priority.