Your MIL was in an accident which has left her disabled and her husband dumped as a damaged inconvenience.
I wouldn't be surprised if she has some sort of PTSD from all of this.
The accident, the shock, her injuries, the pain and adjusting to her new reality, her husband's betrayal, cheating, discarding of her after so long.
Absolutely horrendous.
It's really hard to fathom your husband and sister thinking choosing neutral is anything less than giving their father a pass.
Neither of them have an idea what a shred of decency is to have such a low bar on their fathers behaviour.
Your SIL must have a super low bar and expectations of her own marriage when she finds her fathers behaviour so easy to get over.
If MIL was a good kind and loving mother, she must be further devastated to know that they would so quickly adopt a neutral position.
I have sympathy with you having had a new baby and your husband should be putting you first and telling his father that HE caused this drama, and HE must stay away.
But that would involve your husband putting other people ahead of himself which he clearly doesn't do.
What a shower you have ended up married into.
You do have my sympathy on that score too.