Yes, YTA, and if I may, since you did solicit advice on here, I’d argue that you are the disloyal one. You are ready to scold her because she doesn’t follow your family sex rules? You are ready to turn your back on her for one little thing that you don’t agree with her on? That’s extremely petty. And I don’t think it’s fair of you to call anyone a terrible daughter unless you are the mom or dad of them. And even then, it’s still not ok. You don’t have the right or obligation to speak for your parents. They are grown adults, too. She’s a grown woman and she doesn’t need anyone’s permission to get a hotel room with her fiancé. You have zero control in this situation, and if you can’t make peace with that, you’ll lose a sister. And that would be really sad and 100% on you.