Hard not to hurt anyone.... I Love my dad

I will try and make this as short as I can...
My dad and mum separated and both have remarried. My step mum is truly The Evil Step Mother. My dad is so unhappy in their relationship and has been for a very long time. He says he just can not afford another divorce.
My step mother yells at my dad and they always fight even in front of family - mostly my family. We know she hates us my sister and I, my husband and shows no interest in my children. She has never said it however actions speak louder than words. I feel she is jealous of us perhaps.
It breaks my heart to see my dad try so hard to have a family get together and every time she ruins it. My older 2 children have felt uncomfortable going to their house for a while now because of the way she speaks to their poppy. They come along as they love him and want to see him however now they are saying NO. My youngest child has now noticed that his nanny shows no interest in him and hardly comes to anything and she doesn't speak nicely to poppy (my youngest child's words).
She has 2 children and 3 grandchildren they are not as close as my family and she is not as close with her children as my dad is with his however she does not treat them the way she treats us.
I have tried to talk to my dad and it upsets him when I say I don't like her because..... He gets upset that I worry about him and so at times he doesn't tell me what is going on. I have told him we will support him every way we can if he left her however he says I can't afford it and I don't want to be on my own. I will never find anyone again. Then he says maybe he deserves it.
He has tried counseling however she says she doesn't need it because its all his fault he doesn't listen, he doesn't remember, he does the wrong thing. I'm talking about cooking veggies a little longer than needed by mistake. I'm talking about when he says a story he says on Wednesday and she turns around and says No it was on Tuesday cant you remember anything!
My dad knows we love him. How can I tell him I want to see him and catch up and have dinner, lunch etc, I want him to come to things of my children PLEASE and not let her stop him however I don't want her there. I don't want them calling her Nanny. I never did the only reason they started calling her Nanny was because she yelled and screamed at my dad when my oldest was little because she called her by her name not Nanny so my step mother went off at my dad and he asked me if I would allow the kids to call her Nanny.
Their relationship is unhealthy and it is upsetting to me and now my children see it I do not want them exposed to it especially her. I don't want them hearing how her grandson can do this and that and her grand daughter can do this and that and yet probably doesn't know my children's middle names.
So much for a short post.......
I love my dad he deserves better x