I am really sorry you are ill, and hope that the chemotherapy works.
However, YADBU and forgive me, but it almost sounds as if you are using this awful situation to manipulate things with the ex and the step-kids.
He comes as a package, and to say you cannot have the kids to stay at all for the next 6 months is unwarranted and unjustifiable. Particularly as you have your own DS.
We have a close family member undergoing chemotherapy at the moment, I have been ill for last two weeks so have avoided her. But if I told her that I was ceasing all contact for 6 months she wouldnt understand it.
And why did you're DH tell the kids that they are well and you take priority? That may be the case, but did they need to know that? Surely he could just juggle all the balls on an as and when basis.
Sounds to me like you have issues with the step-kids. I understand that, I am a step-kid myself who didn't get on with my step-mum, but could understand her resentment at weekends being disrupted all the time by my arrival.
Having said that you must have known what you were getting into when you met him.
Please don't use this horrible illness to create/cause more bad feeling, you need to focus on getting better without the politics.
Just sort it on an as and when basis, like the ex says.