OP, this is one of the most useful responses you have here.
Firstly, I am so very very sorry 💐that you are going through chemo, that you have such a thing to face physically, and the emotional rollercoaster and changed landscape. Keeping ok and positive for your kids and yourself is your no.1 job right now.
As for him: he's weak, been weak, will be weak. Make no mistake here - there is now another woman in your life too, and she is organising things how she wants, and he will just say yes. One dimension of pain and shock in this kind of betrayal is how the OW takes possession of him, who is/was yours and your kids', and he will mostly just do exactly as she says.
So basically you are dealing with her.
She is an online sex worker. So she knows about the power of her femaleness. She's monetised it. And she has also used it as a search for a great husband. Yours. She is no doubt fairly ruthless. And that could be such an understatement. (btw not knocking sex workers - just extrapolating basic facts - although online is different to rl because it is so relatively sanitised and actually institutionalised/accepted - with eg public figures and beloved influencers being on Only Fans)
Right. OP. You feel, understandably, totally smashed by this at the moment (another understatment, I know, i'm sorry 💗😓)
But you need to act fast. Because she certainly is. She's not in a dip. She's taking the advantage she has and making the most of it, on a wave of positive energy and forward thinking.
Do what @LadyRoughDiamond says above. All of it.
How did he actually manage to look at you and tell you all this?