Giving lots of background info so there's not a big drip feed, sorry it's long and thanks for reading!
I have a great relationship with my PIL, they were v supportive when I lost my mom just after uni and have always been really kind and welcoming. I especially adore my MIL, we are extremely similar in lots of ways - DH is very close to his mom so she's an important part of our lives. SIL1 is lovely too, we don't have as much in common and she is 15 years older than me but we've always gotten on well and she and DH are close.
The years DH and I were together in uni SIL2 was living abroad, so she was never around when we visited PILs or for parties etc. I didn't think anything of it, DH didn't mention her in any particular way except to (jokingly) say she talked non-stop which is why he ignored her phone calls. After about 2 years she sort of demanded that I be introduced to her in a very 'I'll see what I make of this young one' way. When I met her it was just a bizarre - she referred to me in the third person repeatedly, made weirdly personal comments about my body, was intensely possessive over her parents, her home, her brother... so much so that both DH and MIL told her over the few days to calm down/lay off me.
Since then we've had a sort of surface level polite relationship but it's always been pretty clear she resents me being so comfortable in her family - loads of petty nonsense and drama over the 10 years we've been together but I've tried v v hard to keep it friendly. More recently DH has pulled away and they've had a few rows over her behaviour in general.
Today she rang me to give out that (1) we haven't seen them all summer and (2) she wants to organise a family holiday for MILs birthday in December but no one wants to go (because it will be too much drama). Apparently this is my fault - she "knows" I have told DH we're not going (completely untrue) and that is why SIL1 won't go (?????) It got quite heated, I tried to just laugh it off and say she should talk to DH, this has nothing to do with me etc etc but she was shouting a lot so I just hung up eventually. ANYWAY, apparently my lovely MIL has now been dragged into it all, is very upset and think we should all just go along with it to keep SIL2 happy.
AIBU to say no fucking way!?