Brooke and Katie share the same extremely small social and business circle. The same people get together on every occasion - Eric, Thomas, Brooke, Katie, Donna, Hope, Liam, Marcus, Daisy, Rick, Caroline....Before now Katie and Brooke fitted naturally into the group. However when Brooke appears with her wonderful loving husband Bill who she is all over and Katie arrives, the tension in the room will be huge. They all know the story. They probably all feel Brooke and Bill did the wrong thing. They probably would all feel sorry for Katie. This tension wouldn't worry Brooke of course. I think Bill would be aware of it though and feel some discomfort. It wouldn't matter if Katie was OK everybody would still be wondering what good manners are in this situation. After all they were all at the party where Taylor revealed all. I think that Katie leaving L.A. is a really good option for her and of course she will take her son with her. Certainly she will not even consider giving him up as well. She can make new friends and build a comfortable life where everybody is not jittery whenever she and Brooke are likely to meet.
I think this is the sort of thinking Katie has to do and why she is at Brooke's house and why she cannot have a sister if that sister is attached to her (ex) husband and all Brooke can think about is Panic - She is trying to take Bill from me.