Apologies in advance - this might be a long post...
Wedding is in less than 5 weeks, and OH is on the verge of falling out with his parents forever.
So... OH and his sister haven't gotten on for the last 10 years. She's unstable, unpredictable, has outbursts because the wind changes and never apologises for any of it. She lies to justify her actions and she can be vindictive and uses her kids to get what she wants. The only reason he's ever put up with any of it is for his niece and nephew. His parents continually justify it by saying it's because she's depressed.
They've been pretty civil the last couple of years following a MAJOR fall out where we weren't allowed to see the kids for a year, which was pretty hurtful for both of us. (She bullied my 8 year old niece, amongst other things, and lied to his parents about the whole situation, on OH's 30th birthday).
She was invited to the wedding, purely because we wanted the kids to be part of it.
A few weeks back we started to sort a dress for OH's niece. Just a while flower girl dress, which we will make a sash to match, and a pair of silver shoes. Didn't think it was worth getting anything sooner as she's only 8 and didn't want it to not fit by the time the wedding came round. We sent a few links and asked his mum to order a couple and see how they fit, send us some pictures. They live 2 hours away and there wasn't an appropriate time for us to go up and see the dresses for ourselves - due to them being on holiday, busy, and us sorting things for the wedding, my hen do last week and my birthday (today - yesterday now i've been writing so long!).
We received a couple of pictures of one of the dresses last week, but we weren't keen, so asked for some more pictures and to see the other dresses, which OH's parents agreed to do. OH gets a call out of the blue weds night from his sister shouting at him that he's out of order, she's picked the dress, it's her daughter, she'll wear what she wants her to wear and that's that. Being nasty as per usual, saying she does't have time to run around taking pictures 5 weeks before the wedding (we've literally asked nothing of her up till this point, and will be paying for the dress!)
He was calm and collected, but told his parents that of she didn't apologise this time she's not coming to the wedding, he doesn't want to be worrying about her causing a scene on the day (she did this at his cousin's wedding, in the middle of the dance floor, a couple of years ago).
Well.. his dad called up OH and said it was MY fault because I should have sorted my sh*t out sooner!! And this is where it all starts going wrong. OH stuck up for me and told his dad he was out of order, his dad hung up on him. He told his sister she's not welcome, and she obviously said the kids won't be there if she's not.
Some to-ing and fro-ing involving his parents hanging up when they don't get their way. His mum belittling him telling him to get over it.
OH sent a long text telling his parents how he feels that they always side with her using 'she's depressed' as an excuse, and it's hurtful that they constantly belittle him and have no respect for his decisions regarding the wedding - there was a huge fall out with his parents because he didn't invite some of his uncles that he doesn't ever see, and a bunch of his parents friends he hasn't seen for 2 decades. His dad was upset because his family only get together at weddings and funerals... sorry but not our problem. We didn't have the capacity, end of. They get an evening invite and that's that.
They replied, justifying their actions by laying into MY family.
I've had issues with my sister in the past, but never fall outs of monstrous proportions, and quite frankly, how I handle my relationships with my family is sod all to do with them! They said it's double standards if I'm still inviting my sister.
They also said it's inappropriate to allow my dad to come (who they've NEVER met!!) based on a comment I made about him having colourful language once upon a time - because there will be women and children there. How they fond it very strange that he's not wearing a morning suit like the rest of the bridal party... Again, my family relationships - nothing to do with them!!
Without making this EVEN longer and going into the emotional blackmail they're putting on OH, I can't even put into words how angry I am that they would drag my family into this to try and justify their actions when this is an issue between him, his sister, his mum and his dad.
They also said there's nothing we can do if they turn up at the wedding with her in tow!!! It's not a church wedding and it's not an open invite!
We've gone so far as to check the wedding insurance as this stress has made OH not want to go ahead with the whole thing. (Get married in a registry office and 2 fingers to everyone else). But obviously, this type of thing isn't covered.
Really don't know what to do :'( we haven't slept all week. Both OH and I are affected at work. They've literally tried to control every aspect of this wedding, despite the fact that we're paying for the entire thing ourselves.
OH ended the last call with his parents telling them if they don't hear from him in the next 5 weeks not to bother coming to the wedding 
I tried to calm him down, but he's so upset. I'm upset... I don't want him to regret not having his parents there. They just cannot be reasoned with. I tried to stay out of it right up until the point they started laying into my family.
Sorry, just needed to write it all down. I've gone round in circles (literally, we walked in circles round the estate talking it over) and just don't know what to do for the best. I can't believe they would create such a scene 5 weeks before the wedding...
