Because my MIL is a nice person. Her son *was* a nice normal person but completely F'd up his life with his poor choices. Lost his job, couldn't be bothered to get another one. Started drinking. Then came the neglect of the kids, cheating, and of course, lying about it. And stealing. When I kicked him out he went to live with his brother's family. He actually stole money from his niece's piggy bank. Of course he says he didn't "steal" it because it was always his intention to pay it back.
His brother is also still a nice, normal guy. Hard working, good dad. My ex is still unemployed and basically homeless, but he found a desperate divorcee who took him in. So now he mooches off of her instead of me, his mom, and his brother. Basically he has achieved his life dream of being a male prostitute

I kid you not, he used to say that jokingly.