@Disfordarkchocolate
It's an awful situation but I would say no.
If he moves in you will become his carer, do you want to provide personal care for him?
If he has family he can ask them to facilitate contact.
I agree with
Disfor. Difficult though it is to say "no" in a situation like this, I think it would be too much, physically, emotionally, and mentally for you cope with.
Had he not been emotionally abusive you would still be together. He is continuing to be emotionally abusive now, emotionally blackmailing you regarding the children. AND, it will not necessarily be the right thing for them, because he may start off being OK with you, but he will very soon become true to his type, and maybe even more so as he becomes increasingly ill and afraid.
And as others have said - many cancers are terminal, but not quick. Booth you and your children may have months or even years of him getting visibly sicker and sicker - it will be horrific! (I speak from experience - my DF spent months as a living, yellowed virtual skeleton with his terminal cancer. He was at home, with my DM taking care of him, poor woman, and it was terrifying - and the stink of a sickroom is dreadful and permeates the whole house.
My children, who had adored their DGF, were terrified of him at the end (they were 6 and 3).
He is looking for a carer - and he will very soon start bullying you again.
I wouldn't let him back. Let him find another way to maintain contact with the children.