Don't forget your brother did that pretending right along with her. He didn't take responsibility for his family, or for protecting the kids from her, and teaching the kids how to behave. They
both raised them. She didn't do it alone. Anyway, just a view from the outside.
Okay just read the next sentence in your post-seriously though, your SIL's didn't stay that way on their own. Your brothers didn't stand up to them, so, they basically said "It's okay for you to behave that way, and raise our children that way." I get that it's your brothers, but they are equally responsible. That's all I'm going to say now.
Eww, a licked muffin would more than tick me off.
That is a ruined muffin, that goes right to the trash.
At any rate, I'm leaving for a while. I have my own emotional stuff to deal with this weekend.