If you (the ingame character) did it out of your own desire + knew she was changing and did nothing, why would you tell her family? You would definitely frame it as an accident that happened while adventuring, and Brienne isn't going to mention it because a) it's 95% true + b) she loves you + c) as she's stated herself, she doesn't really care and enjoys being a woman + d) she's a little distant with her family by now.That's what I mean, will Brienne's family demand answers for these things? "Uuh... I may have ignored the signs and then uh, jumped her before she was strictly ready. Can I have your daughter's hand in marriage?"
If you (the ingame character) didn't know, it really is an accident that happened while adventuring.
I don't know why you think there would be a scene where you tell her mother "Yeah, I changed your son into a woman and then raped her on the spot." Why would I write this, and who would I be writing it for?
So to answer your question, no. They won't be demanding answers. I won't spoil the content but they'll just be happy to have their family back.