(((blue))) Could it possably be as simple as she walked into a room where there was an empty seat next to her husband & sat down. I mean it could be inocent. My livingroom isn't nearly as big as I'd like, so for having 4 kids between hubby & I, & friends, I just want everyone to feel comfortable. As long as we're together it doesn't matter in the least who sits where. We can all still enjoy the same space & laugh & have a great time.
Maybe you could try to find a few things about your SIL that you truley like, & start a friendship. Then you might be able to go visit at their house. Hoping you get to enjoy time with your brother before her leaves.