I think that's the definition of holding a grudge.
1. a persistent feeling of ill will or resentment resulting from a past insult or injury.
I hold a grudge against my family as well so not trying to knock you or anything. Your mother might have known a different side of him, or she is willing to let go for probably her children's sake, or if it has become habit then maybe she tolerates it. My whole life was been a series of traumatic events from different family members, my father who was never there until I was 16 and had nothing but negative interactions with, my mother who lost me due to drug issues also had no interactions with until I was 18 - who I just can't relate to as I see her as a stranger. My Aunt, who I lived with my entire life, her inability to adequately care for me with her emotional wall, and my non-related uncle who beat me mentally and physically as well as his actions for example not letting me eat food. So yeah I think I can relate. The closest person I had was my Aunt and when she passed away a couple of years ago, I didn't shed a single tear. I'm not sure if it was the way I was conditioned or if I would have just turned out this way regardless but I have no contact with most of my family for things they've done directly or indirectly. Best advice I can give is just move on with your life the way you see fit, if that includes them great, if not, so be it. Also just a side note grudges are mentally exhausting so if you can forgive then that is probably your best option otherwise I would suggest minimal contact, IMO.
Sorry for the long winded story, and like I said this is just my opinion and the way I choose to deal.