I'm not saying she's right to feel that way, but I am in sort of a similar situation, in that my partner's family spend a lot of time with his ex (they were never married, but together for 8 years).
They socialise together, the 4 women (his mum, his sister, his brother's wife and his ex) and I've never got an invite, despite having been with him for two years now, and regularly visiting them with him.
He and his ex had been split for almost a year when we got together, so I was never the other woman or anything like that.. but for the whole time, the family have made it clear they would have preferred him to stay with her.
It's a difficult one though, as he was treated very badly by his ex before, so got with her when he shouldn't really, as he never truly loved her, yet stayed with her a long time as he didn't want to break her heart, or be on his own.. he tells me he loves me, and he finds that very difficult to say due to situation with his son's mum, so I know he means it - and his family know it, too.
It's not that I think his ex should be banned from the family, she's actually quite a nice lady and I know her myself and speak to her sometimes, it's just that I feel they obviously would prefer me to go away and for the two of them to get back together (they have said something to this effect in front of my partner when we first got together, so it's definitely true).
You go on holiday with whoever you want, and she IS being unreasonable for not speaking to you over it.. but there can be a sort of jealousy and insecurity around exes that aren't really much to do with the ex themselves, but more the position that they hold in the family, compared with the new partner.
I have no idea if that makes sense.