Based on your latest post I think your SIL sounds depressed. I don't think you should call her a bad mom -- depression is debilitating and it really impacts your relationships with everyone.

If you want to help her, I would suggest talking with your brother and mention you are worried she might be depressed. But aside from that you will need to let them figure it out. He needs to encourage her to seek help. Once she's got the depression under control I suspect her outlook will change and she will be a better mother, wife and be more interested in finding a job or going back to school.

Where is she from?

She might be depressed, which can sometimes manifest as anger. If you're close to your brother, talk to him about suggesting therapy for her and suggest parenting books and classes for both of them (frame it as kids are really tough and my own really drive me nuts sometimes, here are some ideas that might help). Try to take the kids frequently so SIL can get a break.