I don't understand. You and BFF SIL don't want to hang out with Big Boobs SIL but now you say you're upset because she (Big Boobs) didn't attend an event you guys invited her to...I'm lost.
Yes, I thought you would invite her if you thought she wouldn't come and were annoyed that she always comes. But even when she doesn't come, she is still the annoying bitch.
It's different when there is gift giving involved. She's expected at those. With a GOOD gift purchased off the registry of course.