Thank you all so much for the replies, I will read them all in detail.
However, here are a couple of answers to questions raised. My sister has never taken criticism very well. Hence, when I saw that post on social media I withdrew from the meal quietly, with dignity and without fuss. It was organised via text, by my BIL, and I therefore sent him a very short, polite text. I really didn’t want the confrontation tbh.
It was later that evening that my sister messaged me to ask why I wasn’t going. I explained why and it all kicked off. My BIL sent the most disgusting messages to me, my mum, my DP. It was vile and I blocked all contact.
For some months previously I had sensed something was off, lots of little things and a sixth sense I suppose, but I can’t explain it very well.
My sister and I have less in common now than ever, we were close but she has become so guarded about everything. To the point where conversation would be awkward and clunky... until she’d had a drink.
I could spend hours analysing everything, to be honest, but I still can’t work much of it out.
However, at no point have I ever said I will never speak to her again, but I feel that until there’s at least some acknowledgement of how hurtful their actions were, I will be back to feeling as though I have a sister who only wants me when she needs something. I very much try to treat others as they treat me. I have a wonderful circle of friends and colleagues, as well as my own DC, to focus on.
I support my parents, I love them dearly and they don’t deserve to be treated like this. However, I also refuse to be drawn into lengthy moaning sessions about my sister, BIL etc, it’s futile and life is short enough! Whilst my parents live close by to my sister, I am 18 miles away. It’s therefore much harder for them than it is me.
Hopefully that helps a little... in a nutshell it wasn’t just the party itself, it was the lies, the weird behaviour, lack of thought, part-time relationship that led to me saying enough is enough. I consider myself loyal and caring, but I can only take so much!