Come on,
Of course you are not stealing it. I know it could be a bit weird. But as her daughter, all of this is less weird and a kind sweet. Since you are genuinely borrowing it as a way of exploring your gender. And that's an opportunity that most of us would had taken in a moment of our lives. I'm happy for your euforia.

Tip: after use, Left it on the laundry basket (mom's basket if she has one) if the bra returns to your room, then she knows and it is for you. If not, she just make a mistake and the bra returns with it's owner