Actually I don't think it's as straight forward as other posters seem to.
When I had a partner with adult children, a couple of them intentionally excluded me from family events and it was hurtful/made me feel bad, and in fact their attitude towards me, and his refusal to hear me voice my distress over it, was one of the many contributory factors in our break up.
Having said that, eight children is a lot, especially eight poorly behaved ones (I only had one, and she was pretty angelic, even the sons who excluded us admitted she was a great kid - it was me their problem was with, or more, just the concept of their dad having a partner of any seriousness, i think.
Is there a path of compromise? For example, how old are her kids/what childcare does she have? Could you invite her to an adult only drinks night or dinner for example? I don't mean too often - just make the effort with her a few times a year. Or, I don't know, let her join the family WhatsApp or something?
You aren't obliged to do any of this, but it would be a kindness, both to her and to your father.
Also, the less you see of her, you do have to accept that this may lead to less of a relationship with your dad, for you and for your kids, his grandkids. This is something to consider too......
In the end families are complex, and nobody reading the bare bones of the story can tell exactly how justified you are or how you should ask. I would just caution that in general posters on Mumsnet are very hot on boundaries and protecting oneself and ones time, and on what you are or are not obliged to do - but don't always consider what it might be KIND to do. And kindness often involves compromise and even a little bit of self-sacrifice, in my honest experience.....