Tbh, if your entire house is homophobic/transphobic, then she could possibly be trying to hide it from everyone else?
It depends on what type of person she is and what tone she used
(The Lil Brothers not thinking its normal is also included in- "I don't want them to spill it to anyone" or something)
If she's not like that, idk, again possibly- she could be transphobic but also could be wanting to keep your trust/not upset you....?
Again, depends on what type of person she is, and this is confusing so there is other options to that