Too young to be in wedding

Hi, so my SIL is getting married and to say the least I have a horrible relationship with her & my MIL. Anyways, so she’s been engaged for 2 years with no planning done whatsoever and last month she called my husband and gave him the date & told him the kids & I (and obviously himself) would be in it. She had never so much as mentioned anything to either of us prior. And honestly I’m pretty irate she never bothered to ask me if it were ok for the kids to be in it before she’s already made plans around their involvement. She has a history of not asking before deciding things about my kids though. Anyways, we live about 600 miles away from her & my husbands family & we see them at max twice a year due to finances, leave, and the fact that his mother & sister have a conniption and demand my husband leave me every time I tell one of them no or set a boundary. (Note: We’ve been going to therapy for 2 years now because of those two, they have an enmeshed, sick relationship with my husband and he has finally snapped out of it and cut the apron strings. However, because he’s put his foot down at their treatment of us and our marriage and children {whom they act as if I am just a surrogate for them to have}, things are more tense and explosive than ever.) anyways, so bomb dropped: she wants me as a bridesmaid, my will-be 3 year old son with aspergers as a ring bearer (even though she already has 3 others) and my will be 1 year old daughter as a flower girl. I flattened her on the bridesmaid offer, since my husband will be in the wedding too, and there is no one there we trust to so much as touch our kids, I can’t be a bridesmaid while corralling two babies who do not sit still, be quiet, or walk straight lines. Not to mention, I’m due with our third in January & that child will only be 5 months old at this time. We will already be spending over a grand between travel expenses like fuel & hotel, wedding gifts, tux rental, bachelor party, food, etc. I’m not blowing the budget further with a 200 dollar dress that makes me look like a swollen celery stick too. I offered to dress in coordinating colors in case she wanted me in any photos (which I’m sure she doesn’t-and is probably glad I turned down the bridesmaid offer because she didn’t seem tore up over it at all when I called her. Now she can pose with my kids and my husband and pretend they’re her little family 🤢) Anyways, all animosities aside, I’m genuinely concerned for my son, daughter & myself. Autism is still very misunderstood, so if we do have him walk and there’s a meltdown (which will inevitably happen; there’s only like 10,000 triggers waiting on him at this wedding.) We will be framed further as awful coddling parents. But even if he were to make it through that, I would have to walk the kids immediately out of the venue to keep them from disrupting the actual quiet ceremony part. With 3 under 3 and one with aspies, someone is always crying. If I walk them out, I’m rude. If I keep them in and they’re disruptive (which they will be) then I’m the awful mom my sil & mil paint me to be on display, helpless while my husband is stuck on the altar as best man. I genuinely mean it when I say that we can’t trust anyone to help me keep the kids there, everyone on his mothers side (which is the only side sil speaks to and will likely be there) all hate me because of what mil & sil have said. And quite a few of the men & the way they’ve behaved around my kids in the past give me serious creepy vibes. And even aside from that, an aspies kid needs to be handled by someone familiar, which would be, but I’m not going to distractedly hand over my infant daughter and brand new baby to someone else just because they’re “family” when we hardly know or spend time with them for good reason. I’m not sure how to handle all of this honestly, I think she needs to choose between my husband and the kids as being in the wedding. If the kids were going to be about 3 years older that’s a different story, but at the ages they’ll be it’s going to be too difficult to singlehandedly (which is the only way it could be done-they aren’t safe otherwise) dress them, get them down the aisle, manage their volumes through the ceremony, and then keep up with them through the chaos of pictures (which they already hate) & the reception (at that point I’ll have an exhausted 5 month old who will have been kept awake and not eating for far too long, keeping a 1 year old baby girl safe in the rush of a party with creepsters waiting to sweep her up and touch her, and 3 year old with aspergers on a sensory overload). My thoughts the easiest way for us to still be there are my husband can remain the best man & the kids I can be there as guests & wear coordinating colors, remain outside for the ceremony If it’s indoors or several yards away if it’s outside, do whatever pictures she wants, and then make an appearance at the reception and leave. That’s asking A LOT from our kids (think they’ve already traveled 600 miles & been away from home for 7 days, they had to go through the chaos of getting dressed in fancy {uncomfortable for sensitive skin clothing}, and not snacking while they wait to be in pictures, the heat and irritation during those pictures, then the chaos of a reception and waiting on all the intros to grab a bite to eat. Not to mention rehearsal dinner the night before, daddy being gone the night of the bachelor party... etc. and then after managing all of that I have to drive us home the next day because my husband will be hungover Sunday and he has to be at work Monday.) Anyone have thoughts on what we should do? Also, the family doesn’t know of our sons diagnosis because even before he was diagnosed they expressed that they don’t believe autism is a real disorder, they strongly stand by the belief that it’s “just bad parenting.” So even if we did come forward and explain it won’t work especially due to autism, then we’d just be chastised further on our parenting.