Elana-
I haven't had this specific thing. But I know I have a really hard time sleeping when I share a bed with anyone. I also get that anxious feeling. Even when I was a kid I found it hard to share a bed with someone at sleepovers. I think you should talk about this in your couples T if you haven't already. Some sort of exposure therapy type work might help. Maybe not in time for her sister's visit, but over time.