Daughter pregnant, son-in-law having an affair.

OP - advice from my extended family where there was a similar situation, whatever you think and feel about your Son-in-law, tone down what you say to your Daughter.

If your daugher and her husband are able to work through this and stay married, if you have been too vocial in your disgust it might make things strained and mean she feels she can't come to you if he does something like this again. And it might make it hard for her to discuss her situation with you.

Other than that, listen, offer pratical support. She might not want her husband with her for the birth, can you offer? Also offer to move in for a week or so after the birth to look after her, if he's talking about leaving, he might not be the most helpful.

Oh my, no advice, except keep calm and get a good solicitor. Maybe make sure your daughter has the opportunity to talk to people who have experience in supportimg women in this hideous situation.
Take each day as it comes, I hope you get the chance to have a private word with the excuse for a human being that is your SIL.
She is lucky to have you.

textuahealing, thank you too, I didn't see your post while I was typing!

Thank you so much for the support, I just had a blip earlier when I could see so many horrible things ahead - my daughter's 33, this will be her last week at work before the baby and now she has this to cope with. Luckily I am retired (early, I'm only 58!) as I have been looking after Daniel while she works for the last four years, but unfortunately I have been delaying a much-needed hysterectomy (for large fibroids) for three years, and had planned to have it done as soon as the baby arrives so I am fit for when she would go back to work again after six months. This was the last thing I foresaw so my slump tonight is probably just reaction. I'm sure with support like yours I'll feel better in the morning, ready to get on with it whilst keeping my mouth shut. Right now I could murder my son-in-law (but I promise I won't!)
Thank you again, even just one voice made a difference.

Sorry to hear your news and your daughter is lucky to have such a supportive, loving mother such as you. I think you are maybe wavering between needing to stay calm and focussed on your daughter's well being to wanting to rip your SIL's head off his shoulders. I've no advice for you but wanted to show support and feel anger on your behalf. (Don't really know what type of person the OW's mother is to turn up at a home with a heavily pregnant woman. Just goes to show what type of person, he has got himself involved in!) Best wishes you all.

I'm so sorry to hear your terrible news. How shocking for you. I am sure you are devastated.
The sad thing is, as you correctly assume, your family's situation is by no means unique.
I am probably of a similar age to your daughter and several of my friends have experienced marriage break-up in similar situations. One school friend's situation sounds almost identical.
It turns out that most domestic violence and affairs begin during pregnancy. Apparently some men cannot cope with the jealousy they feel about their wife's affection being focused on their child.
There is not much I can say, except that you have all the right ideas. Give your daughter as much support as you can. She must be scared at the idea of becoming a single mum in such circumstances and also sad that the break-up of her marriage. However, with love and support from her family and friends, she can rebuild her life.
Good luck.