Issues with my sister in law?

First be honest with her and let her know that you don't mind hanging out with her but you would prefer not to talk her relationship with your brother. Let her know that it makes you uncomfortable. She has a lot of growing up to do, you can tell this by how she sends your nephew off for the weekend. She wasn't ready for a full time commitment to being a mother or a wife and she is heading for disaster. Someone will wake up, either her or your brother. Maybe suggest to her that you would like to help her clean the apartment so that your brother can come home after along day of work to a clean place. If she takes offense, let her know that you only want to help. She will either get the message and take your help or get mad. If she gets mad than you know she feels guilty about not keeping it clean on her own and maybe that will be enough to send her in the right direction. Good Luck