I would ask her how you can help her during this difficult time. Perhaps she just wants some company, maybe to talk or watch a movie or maybe just to not feel alone.

Perhaps you could do errands for her like picking up some groceries. You might make a casserole or lasagna or whatever they like to eat since she probably has less energy to cook now. Or prepare a picnic she can share with whoever she wants. Ask her what she would like.

I am not a very good artist but I like to make homemade cards that seem more special than ones from stores. Maybe buy her candles or bubble bath or a music CD; whatever you think she would like. How about a manicure and spa pedicure?

I am sorry your sister is ill.