Some things that I've found people in this sort of situation appreciate:
(1) Offer to do errands. You can call and let her know that you'll be doing some errands the next day, and ask whether you might pick up anything she needs at the places where you'll be, or make a stop somewhere else while you're out. If she declines the first time, call again another week. She'll have time to think about it and realize things that it would be helpful to have you do. If she takes you up on it, call her and let her know places you're going and see if you can get things that she needs there. Don't pay for the things for her, or she'll feel awkward asking again.
(2) Invite the children over to your house and on special outings occasionally. This helps her and the children. She can relax knowing that they are in someone else's care and are having some fun, and they benefit from being out of the house where sickness is bound to put a damper on things, and lets them just be kids for a little while. Their mother probably has little energy for taking them anywhere fun. Let her know that your kids enjoyed it so much more having her kids come along to help her feel less like a charity case.
(3) If the husband is overburdened taking care of her, consider having your husband (or teenager if you have one) come and mow their lawn or rake their leaves once in awhile to take the load off. This will free him up a little to give her some more attention or have some free time for himself.
(4) Offer to help put out their holiday decorations at Christmas time, and then take them down afterward.
(5) Food is always a thoughtful gesture, but sometimes people with medical conditions have diet restrictions or find only certain foods to be appealing. On the other hand, feeding the rest of the family helps as well. Make it easy on them by using a disposable container and making something that can go in the freezer for use on a night when cooking will be difficult.