Okay this may take moment to lay out the details so bear with me. I need some advice on how to handle this.
My FI and his sister have never been close, and the relationship with his mom has always been strained (just oil and water) but his sister and his mom get along like best friends. When we got engaged, they gave him the third degree about the decision, alluding to him deserving more, me being a gold digger etc despite him telling them how he feels about me and how great we are together. They have always held only surface conversations with me - never getting deeper than 'how's the weather'. That being said, family is very important to me and I thought it would be a nice gesture to include his sister in the wedding as a bridesmaid (just her and my best friend as the MOH) and FI was all for it - he thought it was a really nice idea. We had a few glitches with his parents after the engagement and it all culminated in a big conversation we had with them a few weeks ago where we sat down with his parents and for the last and final time justified our relationship to them. So, we thought everything was fine and dandy.
About two months ago, I had talked to his sister and his mom about going BM dress shopping on January 4 - my mom and MOH would come up and meet with them plus my two daughters and I (6 people), and we could all go out for lunch together (since it's my birthday too) and have a girl day getting to know each other. I watched as it was marked on the calendar and we even discussed it last week at christmas. Fast forward to last night when I messaged everyone to let them know that we had the appointment confirmed for 2pm and to see if anyone had ideas on where they'd like to go for lunch. I got a message back from my FI mom on Facebook asking me to change the date because she wants to go shovel snow at the cottage that day because it will be the warmest of the week...now, keep in mind that FMIL and FFIL are both retired (and my FI has been off for the past two weeks and they have 'tenatively' scheduled to go up with him twice in the last week but cancelled) so they can do this any day. My response that I sent to his mom was 'No I can't...if you can't make it that's okay. I already have everything arranged for that day and have my mom and MOH coming up from Newmarket to have lunch for my birthday too. If FSIL can come, great, if she can't that's okay.' She then tells me that FSIL is going with them (even though she is not physically able to shovel snow off the roof) to help and sorry for the inconvenience.
I told this to FI and he lost his shit, threatening to cancel the whole wedding because it's just too much bullshit and drama. He called his mom, had some not so nice words about the whole situation - it was for my birthday, a nice way to get everyone together and introduced, and included...not to mention it had been on the calendar for months. He told her that unless she hears otherwise from us, the wedding is off. I have since talked him down off the ledge and we have decided not to cancel but rather that to avoid any more drama it would be best to tell his family that from this point forward they will not be asked to do anything wedding related. They will receive their invitations and are more then welcome to attend but that they are no longer going to need to participate in any of it. This includes his sister, we feel it best that she not be a bridesmaid at this point. How would you ladies approach/word this conversation?
Secondly, my brother is acting as a groomsman and now my side is uneven...his girlfriend (who has been around for years) would be my first choice to step in and I want to know if/how I can approach her without her feeling 'second'?
ETA: FSIL is 30, and has not said anything. She has her mom make all the arrangements on her behalf because they are 'that' close.