Something I'm not understanding is how people think it's okay if OP is uncomfortable. If the sister is showing off parts of her body that OP isn't comfortable with, OP should be allowed to ask her not to. Of course, there's different kinds of sports bras of varying coverage, but the booty shorts means that a decent portion of her bare ass is at least visible. OP did not consent to seeing that. I wouldn't want to see any of that from a SIL in common areas, and I'm a girl. Changing in front of an in law of the same sex? Sure, had to do it before, but lounging like that? Bathing suits are another matter. Like, sure, it's basically the same thing, but if I'm uncomfortable in the house that I'm paying for, it isn't on me to learn to cope in my house. I should be able to choose when and where I'm exposed to things that make me uncomfortable so that I can learn to deal with it on my own terms.