As lots of people have previously said, I think that the exW's presence is, on balance, great for the kids and natural for the family as a whole - whether they have worked hard to get there or it just happened easily doesn't really matter. My point is that it isn't likely to change, and almost certainly shouldn't.
Instead of focussing on what the ex does, maybe there would be a more positive outcome if the OP focussed on what she could do. Maybe one way the OP could make herself feel more comfortable within the wider family if she built some closer ties there for herself. OP: could you make the effort to become closer friends yourself with your partner's parents, and/or with other members of the family - does your DP have a sister you like, or a cousin? Build friendships of your own. And maybe you could host family gatherings at your home occasionally, or suggest a meet up with your DP's children and their cousins in your local park - if these gatherings were on your own 'home turf' maybe you would feel more confident. Plus, of course, the very act of hosting gives you a key role in the event, which would probably help you to feel more central to the gathering.
I'm not suggesting that it all has to be big events and grand gestures, but little actions over a period of time. The chances are that you'll become more comfortable without even really realising it. In time, you may feel able to host a whole family event and even invite the exW yourself.
From what you say, it sounds as though your DP's family are generous and welcoming, and I would guess that if they felt you making that kind of gentle effort to forge deeper relationships, it would probably be instinctively reciprocated.
Could you do that? See what you can do to change the way you feel, rather than what the family should do?