I'm sorry 💐it's a horrible, horrible thing to go through, but it's honestly not the end of the world - even though it feels like it
As others have said, he's no longer your friend. That's very difficult to get your head around and it's easy to think 'yeah but he's different, he loves me & the kids and will do the right thing' but try hard to believe that he isn't any different and you can no longer rely on him to do the right thing by you & the kids.
Mil - it's lovely that she is being nice & is upset etc, but at the end if the day, he's her son and he will be 'forgiven' and while she might be great & try to stay being your friend etc she will eventually move away from that and embrace his new life. It's hard, it hurts, but it's the way of life.
I'm glad you've got your mum, though be careful too, they often want you back together...no matter what.
I hope you have a good friend that you can cry on, who will support you but also be straight with you.
It is a shitty shitty time, but you WILL come out the other side!
& yeah 'nothings happened' is BIL licks, but you know that!