I haven't watched the special features in a long time, and haven't listened to the commentaries, so I don't know for sure if it's true. She stripped down in TRUE DETECTIVE, so I'd say it's possible she was willing.

Maybe the film-makers thought her breasts being out when Leatherface shows up and notices her scar, it would have been a little weird because they're cousins or technically he's her uncle. I don't think the film ever got that straight. Just another example of either bad writing or uncomfortable incest winking.