Great sister! Please tell everyone. You'll need them. It's tempting to keep secret and private; hurt, shock and embarrassment can make us retreat to hide, but support from everyone who loves you will be key.
Just be careful how you word things - you don't want the 'see what I had to put up with, she's crazy' narrative to get any fuel.
I'd stop speaking to him at all, other than childcare business, and avoid being around him. He's already twisting what you've said and done since the news.
Again - legal and financial advisor appointments today. You may be surprised what else you'll learn about his true character soon, and you'll need them too. All legal and financial advice needs to come from them, not Mumsnet!
Tell his parents today what he's done. He will be painting a different picture. They need to know the devastation he's wrought. A frosty reception should be waiting when he gets there.
Ask them to keep your DC with them if he goes to meet her, dont let him take him along. ( I wouldn't be surprised if he intends this.)
And because I have a very nasty streak I'd be messaging his line manager too 😈
He may find when he is facing the reality of living with a young stranger and her children who loathe him, plus furious parents nearby and his own children 300 miles away that having is not the same thing as wanting.
Again, beware the scuttling back likelihood. I would strongly advise you to finally discover your common sense when it comes to what you'll put up with from him, to decline and put up the barriers.