Sorry you're having to go through this OP on top of your own illness. It shows in your post how draining it all is for you.
Teens can be very frustrating! But if they've not really had to do much round the house before, this is likely to be a big step up for them. Their Mum is ill and they are having to take on more responsibility - two big changes at once. Teens are not like young children, but they are not always emotionally mature enough to cope well with situations like a parent being ill either. Teenagers facing a parent's serious illness can feel scared, like the rug has been pulled from under them, resentful, angry, insecure, unsettled - and may not know how to express those things.
Parents also do often expect things of their kids at such times that they have never really been taught to do or navigate, and may expect them to react in a more adult way than their years and maturity. Back in CAMHS, I've had to have a gentle word with many a parent about this - they are hoping for their teens to step up and go "whatever you need mum/dad". Which of course is understandable - but that's also not realistic, because they are not adults, and they still need their mum/dad.
It sounds like they may need a family meeting and for you to help them with some structure, such as a rota, making sure the jobs are divided up fairly between them and making the expectations and boundaries clear. What jobs need doing, how often, when do they plan to get them done, what are the consequences of not doing your fair share. And they may need the effects of your treatment explicitly spelling out to them. They will not automatically know that radiotherapy will make you sick and tired.
It may also be important to schedule a day off for each of them as well, where they get to not have to think about chores and caring. And you may also have to accept that in some areas, standards may slide, because they may not do things as well as an adult will do them.