Thank you all - even the extreme suggestions! Didn't sleep very well, but was expecting that, and some of the things posted have really helped to reinforce what I was already thinking.
SIL says he wants to be at the birth, but that's up to my daughter, she's what matters. Am not looking forward to seeing SIL next time, will have to be very strong, and neutral. I'm angry about the occasions my daughter was out and he wanted me to babysit because he "had something on" - yes, now I know what it was....but this isn't about me.
And there's a bewildered 4-year old in the middle of this, things have to be as normal as possible. I'm not quite so worried about the baby, he or she will be born into the situation. At the moment I don't think my daughter will want the marriage to continue, but as you say, there's always a chance, so mustn't do or say anything now to jeopardise that. One day at a time is such a sensible thing right now.
I didn't realise what a huge help this site was going to be, took a chance really, but thank you all so much.