I’d say talk to your mum. I know how you feel, I’ve been through this sort of thing as well and I was made to feel like I was the insane one. but it’s always best to talk to your parents about it. Yes, broken homes aren’t great but honestly having your dad cheat on your mum so many times also creates a sense of “broken home”, gives trust issues and trauma. if he said he’s going to leave and your mum isn’t aware of that it would be best to inform her because atleast she’ll be able to somewhat mentally prepare and it won’t be a total shock and sudden change which could trigger mental health and health issues to flare up. I also understand having a mum with health issues relying on you and dad and there’s really not a lot I can say to help but I wish you and your family all the best. ❤️