Spouse keeps threatening to leave my father, any advice?

Looking for advice. My father has mild to moderate dementia and a previous brain bleed. He's not able to do what he once was, but he really lacks awareness of his limitations. My stepmother of 40+ years gets very angry with him and verbally abusive. Several times now she has threatened to leave him. He's 80-years-old. He had a brain bleed. He's slow. I feel like she's stressed with how much care he needs, in denial about how bad he is AND also acting like a spoiled brat.

This week she yelled at him for not managing his time well and making them late. He gets yelled at if he burns something in the microwave or if the dog slips outside when he opens the door. He's doing the best he can, in my opinion. On the flip side, he IS very stubborn, takes no responsibility since he can't remember what he does and can really be a handful, too.

Last year when he had his brain bleed I told my stepmother most brain bleed patients only live a year. She recently mentioned to me "it's been almost a year now" and I felt like she was subtly complaining that he wasn't dead yet.

Of course I have several siblings who are not involved at all. Meanwhile I have made over 80 trips for groceries, doctors, veterinarians, etc. since the pandemic began to help keep them safe, since they're both high risk. And that came right after helping him recover from the brain bleed. I know my stepmother is overwhelmed and frustrated because she never gets a break. But I really want her to suck it up and stop threatening to leave.

Whatever happened to "for better or worse?"

P.S. The brain bleed was a really difficult time. I had to implement a lot of in-home care for many weeks, and they were both angry and rebellious about it. But eventually it was all stopped once he recovered enough (Meals on Wheels, physical therapy, occupational therapy, shower helper CNA, etc.) They're really resistant to in-home help, yet my stepmother feels put out by having to cook dinner for dad.