YTA 110%. She is a grown adult. She completely respected your parents and their house rules by asking permission. When she was denied, she found other accommodations which is absolutely her right as a grown adult. You, on the other hand, were super disrespectful by butting in on their conversation with your sister as if you have a say in their household rules.

She is not having "casual sex". She's been with the same partner for 8 years and has been completely committed to them for 4. Sounds like a completely normal adult to me.

You're not only an asshole for judging your grown sister for things that only affect her and her fiance alone, but I think you've reached flaming asshole status by not minding your own business in someone else's house. What goes on in your parents house is their business, not yours in any way, shape, or form.

I suggest you dedicate all this energy to something that actually concerns you, OP. Your sister is better off without your unsolicited opinion of her life.