Quoting Rebecca Hasbrook:" I have been saying this for YEARS! Lol. She would even say I'm finally done, I'm leaving him.. But ... [snip!] ... to be gay.. I'm just worried about him and being in school.. Kids are MEAN! It can't be easy to deal with His parents though.."
Honestly, everyone has their limits. I'm not on HIS side AT ALL, but it's what your mom is willing to deal with, so he sees no reason to change. My dad was very abusive to all of us too. My mom left many times and always came back. We also learned that he was very mentally ill, after my mother had put him in jail, and he had attempted suicide.
Now that my dad is on meds, they are very happily married once again, and no more abuse. (Thank God)
But, like, lets say, how your SO was to you, I wouldn't deal with that, but you have a different limit to what to put up with. Just like, I'm sure you wouldn't deal with some of the junk my husband does.
I hope your mom gets out of this situation soon.