Your husband and parents have the right to draw boundaries at the doorstep of their home for any reason they wish.
Unfortunately, the situation asked for only a simple sentence of explanation. Your husband could have just said "Sorry, I'm going to come down on the side of caution, you can't come, we're at risk." Unfortunately, he threw in "your mom is stupid," and judgement of them for what they do (which of course every teenager loves to hear about themselves), and you're writing for advice about how to get them to acknowledge they are wrong. This is a more emotionally loaded argument than the situation called for by about a thousand percent.
Make the plain statement that they can't come because you guys fear exposure, and learn not to care whether they think you're wrong. They can think whatever they want, the goal is not to have them in the house.
This made me cringe, to be honest.
To be fair, I'm with you. I'm super cautious, and wear a mask every time I leave the house, and thankfully, I'm working from home and that's not a concern. I'm also higher risk with heart conditions.
Your daughter just finished chemo and has already had covid? Chemo wrecks her immune system, and she shouldn't be going out and about like that, covid or no. Can you talk to her doctor and get the recommendations for her?
Whatever the recommendations are for your daughter, your son should be following, for your daughter's safety. (Also, of yours, your husband's and your husband's parents.)
Normally, they may have antibodies that may offer some protection, but undergoing chemo may change all that, and no one knows how long those last.
"I won't be a prisoner in my own home" is the statement of a child, or someone really selfish, entitled and immature. I'm sorry she can't see beyond her own needs to protect the rest of her family. For now, personally, I'd say no visits.