I will sit through a service I am not familiar with (remember I'm a different faith) by myself as DDs will probably sit with their friends (they mostly go to that church) and MIL will sit with Mary FAy and her entourage. I will be very uncomfortable to say the least.
I'm sorry to say that I don't really have a lot of sympathy with this point.
You just sit there. You stand up when other people stand up, and you sit when they sit. And if there's singing or responses, and you can figure out what's going on, then you participate or not as you wish.
It's going to come out cold when I say it, so I'll try to say it as encouragingly as I can.
Etiquette requires us to suffer through situations in which we are uncomfortable.
Growing, as a human being, requires us to *learn* through situations in which we are uncomfortable. Want to get better muscles? You have to work out, thereby actually *stressing* those muscles; in the process of repairing all those little injuries (micro-tearing of the muscle fibers), the muscle becomes stronger.
Being uncomfortable is something that we all just have to put up with. It's not the end of the world. Nobody's going to bleed; nobody's going to suffer physical pain or even extreme emotional trauma. It's just a wedding. That's all.
You aren't even required to participate in the worship service. You just sit there and watch--that's all wedding guests are expected to do. At weddings far more than at other church services, you just sit there quietly and witness. Even at Catholic wedding ceremonies, you can just sit there. You don't take communion if you're not a member, and you don't have to say all the responses, etc., if you don't know them. You aren't invited in order to worship--you're invited in order to witness.
There will be people at that wedding that you know--that you are related to by marriage. You will not be isolated and stranded and ostracized. Maybe these aren't your closest friends and relatives; maybe your relationship
with your DH's family is sort of strained. They'll be perfectly friendly.
And you have a 4yo, and you're a VERY experienced mother (witness the ages of your older children), so I wouldn't expect you to really need your DH's help in terms of childcare. 4yo's are really pretty well behaved, and you'll be occupied explaining things to him, or conversing quietly with him before, etc.
I think you just don't like the bride's mother, aren't a fan of the family dynamic, and so you're coming up with all these other things to justify the idea that you have a right to be angry with them.