Its different when a friend has cancer than when its a family member. When you live in the same house as someone with cancer, you need to be a bit more proactive and protective than when its not someone in the same house.
For the person with cancer, they are probably feeling a bit stressed, to say the least. They may isolate themselves or they may act in an uncharacteristic manner - drinking more, eating more or less, etc. Their whole being is probably focused on their diagnosis and how they feel, which may or may not be very well. They are contemplating their odds, their upcoming treatment, etc.
This is not a time to make undue stress on them. They need their time to regain some emotional equilibrium. They do not need reminders to take their medications, eat food, or interact with people. You can ask once but do not nag or stress them. They are not in a good place.
The most important things that they need right now is support. They may need trips to the doctor. They may want help finding more research on their health.
They do not need aggravation. Trust me. I've been there.
Family relations can be complicated. This is not a time to complicate them further. This is a time to uncomplicate them so quit all the drama.