D.H.
That is very nice of all of you. Having been in a situation where there was a long term illness I can say all help and visits are appreciated. May I suggest going beyond food, into yard and house work, running errands and sending Mom out for a spa day about 3 weeks into it? DOn't ask what you can do...they will probably turn away help. Just go DO it. Call and ask when a convinient time is, then go in, tell her to sit and go strip her beds, clean her bathroom and kitchen and fold her towels. Get a group to put up their X-mas lights. Bring crafts or stuff for mani-pedis for the girl if she is up to it. Maybe you can all learn to knit or crochet and make her hats as a project with her. Bring the fun to her....movie night? Bring soda, popcorn and drinks. Take the dogs for a walk, help with the other pets if there are any.
Most of all, keep it up and don't overdo. Once or twice a week and rotate duties...maybe one of 2 meals a week. This is going to go on for a long time for them and they will need support after the new wears off. SO pace yourselves. Send cards, leave notes around the house for them to find with encoraging words. Don't forget all of the family, take the other kids (if there are any) out too.
Good luck, I hope she fights and beats her illness.Remember not to let this define her, or them. Your job is to hepl with the normal.