Hearthealth, both my parents have cancer. My sister lives twenty minutes away from them (I live an hour away) but refuses to pretty much do anything regarding their care and treatment. I spend too much time wishing that my sister would become somebody that she's not.
The final straw for me was when I told her that my mom had threatened to commit suicide (she had decided to cut out her anti-depressant which was prescribed to her once she was diagnosed with cancer because she thought it was making her go to the toilet a little too often - fast forward two weeks later...) by walking into the pool and I had to call in a psych evaluation. My sister said "I'm sorry, but I can't help but start laughing." Her voice was so cold and so detached I literally felt a chill run up my spine. And then something in me just snapped.
I gave myself permission to no longer tolerate her victim mentality. If she took offense over something that I said, I didn't try to fix it. One thing that I do sometimes is replay a conversation I have with my sister, but I swap her with a normal person. It prevents second guessing myself, and it reminds me that if I have to interact with her, I need to walk on eggshells. Since I don't want to walk on eggshells, it's best for me to minimize any interactions I have with her period. So I no longer keep her updated on my parents' status. I DO spend time with her daughters, and just this week, I told them what was going on with their grandparents because she couldn't be bothered to tell them herself.
I hope you grant yourself permission to no longer tolerate his BS so you can concentrate on your mom and your kids. It's draining, and it takes your attention away from what matters most. Be there for your mom, be there for your kids, but in regards to your AH, be aware that any attention you pay towards him is energy that you are flushing down the toilet.
And cancer sucks.