While I think that you have a justifiable reason to be upset, will hanging around the house help or hurt? Flimsy excuse, "My S is going to be uncomfortable". Whatever. Plan something fun and do it.
I'm not familiar with your sitch. If you're still sleeping at home, I think it's unfair of her to have to make plans to sleep elsewhere because her sis is in town. Maybe draw the line there -- "I'll be gone for the day and evening, but I'll be sleeping here, though I will leave you two alone so that you can visit."
BD
My latest
Me: 36 W: 35 2 D: 9 and 5 T: 16 years M: 12 10/4/06: Bomb 10/5/06: Ended A 4/22/07: ILYBNILWY