I think your hurt response, while understandable, especially given your losses, has, over time, come to victimise you. Originally it was your bil and sil who were wrong, rude, petty, pathetic and difficult to deal with. Over time your refusal to see them until you have received an apology has turned you from being the people who were wronged, to the people who are being difficult to deal with. I’m sure you never envisioned nor wanted that.
Just as your SILs behaviour has affected everyone in the family, so too now, is yours. So while you are angry that the PILs didn’t discuss this with you first (which I understand you feeling), I wonder if they find it as difficult to talk about with you, as you do with your SIL, and what they are doing is just putting their wishes out there in the hope that someone in the family might find a way out of this problem.
Because they probably feel they are being hurt by all the conflict too, and just as you feel hurt and victimised, they probably feel helpless too. So just as you are entitled to put forward your request that you receive an apology, so are they entitled to put forward their request that everyone puts it behind them and gets on. And just as your SIL can say ‘no apology’, you can say ‘no tea’.
It’s really complicated and difficult for you, but you need to ask yourself, how do you want this to continue? Because unless you can resolve it, it will continue and get worse. I imagine soon it’s going to affect your relationship with your other in laws who you get on with, who will also have a difficulty with choosing what to do.
Your SIL is clearly batshit and no wonder you don’t want to deal with her. And you could have chosen to swat away the accusations, by ignoring them completely and making her feel silly and marginalised (and everyone would have agreed with you) And probably forgotten about it and focused instead on the absolute joy of your having a child after several miscarriages.
But the response you chose, while seeming to be sensible initially has not served you well over time. It’s given the issue legs, drawn all the family, including the children in to the conflict and slowly allowed SIL to seem like the reasonable one (because she doesn’t want to talk about it) while you have got stuck in a no-win lane of demanding an apology which clearly you are never going to get. (I’m sure she loves that!) If she is never going to apologise, what are your options as more people get drawn in and become victims and end up feeling resentful just like you?
Insisting on a formal apology is a bit like submitting the person to a ritual humiliation, a ceremony of ‘I am right and you were wrong and now you admit it’. That’s very difficult for lots of people, they will never agree to that. Plus everyone knows she’s wrong and crazy already (we all think that here!). Plus, you can’t rely on some people doing the decent thing, and your SIL definitely seems to be one of those people, so in that case you have to get smart to get even. Clever, subtle steps… Formulate a way of dealing with her without drawing everyone into the conflict, which is damaging for you over the long term.
Like come to tea with something to undermine her so, so subtly, like a drawing of the family tree which in its own way will show how many people have the same names over generations and casually show it to the kids. Something like that.
Let her occupy the batshit corner of the family and don’t join her in it.