But see, that's the sister's problem there that you've outlined. Not the ex's.
Family relationships are littered with "divided loyalties" all the time. The best thing for you to do is (IMO) simply make your own relationship with them and don't worry about hers.
My STBXH seems to have this weird idea that MIL can't have any type of relationship with his OW if I'm still friendly with MIL. It's ridiculous. I don't give two hoots of MIL has a friendly/good relationship with OW. I suspect it's strained at present because, as I said, of their behaviour and MIL's distaste and irritation about that. But I am careful to make sure that MIL is not put in any position that will make her uncomfortable - so I make sure that I don't go to MIL's when STBXH is visiting her or staying with her. STBXH seems to have no qualms at all about her being in the middle, which frustrates me to no end.
It sounds like your DH's ex is doing nothing wrong here but you placed the blame squarely on HER in your OP... for example:
Ex wife won't leave husbands family alone
still invites some of them to major family events with her family such as Christmas and Easter and they attend if free! In fact I had a very hard time being accepted by his family which I am sure is linked to their relationship with his ex
I just don't know what the motivation is to remain so tightly knit with my husband's family and just indicates to me that she hasn't moved on after all of this time
I have said not to as thought it would seem bitter and make it look too much like we care and that we should rise above it and just ignore. But it does bug me! We are spending Easter alone and his family is going to the ex wife!
But then you've just said this:
I don't think I see the ex as competition, not at all and am not jealous of their relationship
I think it's pretty clear that you DO see her as competition and ARE jealous of their relationship. Until you get past this, you are going to have problems with it. Both she and his family are free to choose to socialise with each other - they've obviously been socialising as family for many years, still have family members in common (the children), and want to continue socialising. There is absolutely nothing you can do about this, and to be fair, there is nothing wrong with it.
You need to forge your own relationship with them on your own level and not keep comparing things to the ex.