Thank you to everyone who has shared their perspective. The questions asked are really helping me to see what it could be. I've taken a long time to reply because I wanted to read lots of people's views before responding. Sometimes on these threads everyone says the same thing on the first couple of pages then different perspectives come through later that can shed different light on the issue. Also I have been out for the day with the DC and didn't want to read the thread whilst with them because it would bring the reality of the situation back to me and make me not very good company.
Answering some questions:
No I don't have a drink problem. I drink the least but not in a judgemental of their drinking way.
We're all full siblings and our DPs are still together.
We live close enough to drive there and drive back in an evening. DH always offers to drive so I can drink with them.
I'm really accommodating with meeting when and where suits them. When the DC were little I gave parameters because of naps but otherwise would make it work.
There is a wealth gap with me the poorest. I chose a vocational career which I absolutely love. They chose corporate highly paid highly stressful roles (although are very senior now so less stress in a way). I'm not bothered that they earn more than me because I made the choice to be happy at work rather than earn a lot. I know you can have both but the corporate career I started in was relentless so over a decade ago I switched out into non corporate and have never regretted it.
Looking particularly at @WhereYouLeftIt explanation. I am the black sheep/scapegoat and have been since I was a child. But I worked really hard at not being controversial or hard work. Hence I now have lots of friends. I'm consciously that nice new version around them because I am acutely self aware.
I feel disappointed rather than angry about this. I'm upset because I don't know what I've done wrong because if I knew I definitely wouldn't have done it or if I knew I would apologise for it.
I'm not more successful in comparison to them. I'd go so far as to say my DM has been embarrassed about my comparative lack of boast-able success in the past. I'm doing what I love, find meaning in my work and can afford a 'normal' lifestyle. By that I mean a mortgaged house, long term doer-upper, bog standard car, 1 foreign holiday a year in a European destination - all different from my new Tesla owning, multiple foreign holiday going, mortgage free siblings. But I'm genuinely not envious because I don't want to do what they do for a living to earn the kind of money you need for those things.
I lived two hours away for a couple of years in my 20s but deliberately moved closer once my Sisters starting having kids so I could be nearby for every family event. In the 20+ years since I have never opted out of one for any reason other than simply not being available e.g. holidays, important prior engagements. I've always bent over backwards to accommodate them and DH fell into line with that when I met him. It was taken as read that's what I needed/wanted to do.
I decided to cancel my Monday with my DM. The weather isn't looking great so I've blamed it on that and suggested other dates later in the holidays to buy me time to decide what to do.
With regards 'there must be more to this. What are you hiding?' please do ask me any direct questions about what might have happened in the past or things I may or may not have done. If there was something obvious to me e.g. my political views then I would include it to help people help me make sense of it. But as I say, I am very aware that there are topics to avoid so I do.
I'm not vegan or over/underweight. I'm neither beautiful nor successful. I have 2 wonderful DC 1DS and 1DD. They are easy to get on with when they come to family events. They are interesting and interested (early teens). I knew DH was the one for me because he was the first of my boyfriends to really get on with my friends and family without any awkwardness. He's easy going and remembers things about people and asks them e.g. how their ski trip was.
I get on particularly well with my DM's Sis who I'm meeting with my DC over the holidays. She was there at the DM 'spontaneous' lunch when I was excluded and afterwards apologised on behalf of everyone who was there and didn't think to invite me. It wasn't necessarily her apology to make per se but I really appreciated that someone there saw it was wrong and apologised that it had happened. That validated my hurt that time.
My Bro missed a Christmas get together last Christmas because of a last minute holiday with his then partner. Sis50 was really annoyed with him not being there for an event she'd organised. But there he was at her 50th not me! (Who was there at Christmas).
This point by @Calliopespa is really helpful in helping me understand the situation more clearly.
I'm sorry but for some reason they don't like having you there - and the feeling is not limited to your Dsis. None of us can say if its justified or not from what we know, but the fact it isn't just your Dsis and the fact they don't enjoy you being at these functions is plain.
You are right. They don't want me there. Which helps make my decision not to be there in future. Knowing they feel this about me would mean I would no longer enjoy it. And knowing they would enjoy it more if I wasn't there makes things so clear. Thank you.