I’m so sorry to hear this OP but at least you have irrefutable proof.
Get ahead of him. You are one step ahead now, keep it that way. If you aren’t up to this yourself, Give your brother all your details to allow him and SIL to help you
withdrawn 50% of money from the joint accounts so you can’t be screwed by him getting there first. In fact seeing as you’ll have 2 kids, take 70%
change the locks on the house
submit a CMS claim stating he has no overnights. I would not be allowing him any with the three year old, so you can maximise CMS at this stage when you are going to need money. He lost his right to easy access to your children when he had the affair.
Do not put his name on the birth certificate of the second child (this will not affect child benefit or CMS)
apply for universal credit tomorrow as a single parent - you need to be financially secure to help you play the long game of battling him being a dick, because he will be when he realises he’s lost the game!
ensure child benefit for your first child is in your name
file for divorce
all these applications can be done in a day if your parents keep hold of little one and your brother helps. Your head will be a mess, but fight fire with fire! Be ahead of the bastard!