So it is a tiny house and you are basically fuck buddies? I feel like he needs to keep his fuck buddies out of the house at least until she adjusts. It can be really, really hard to adjust and sometimes adding in randoms like that makes it even more difficult.
It is most definitely NOT your place to say something to her especially if you are going to accuse her of cutting which it could be, but it could also be something completely different. You are a random "special" friend for her Uncle. Not a family member, friend, custodian, anything. If anything, be friendly to her and let her know she can talk to you. I don't know the exact dynamic to know if that would work, but that is the most I would do.