Well, if you're a praying person, that's the number one thing you do for her. But what else?
Communicate! She may not be able to reciprocate, but keep on letting her know she is always in your thoughts. When people are seriously ill - ill enough to be in nursing care - their family and friends sometimes write them off. It's not necessarily intentional, but folks don't quite know how to handle that sort of thing, so they may just let it slide. Don't do that. Let your mother-in-law know she is still valuable by sending her letters when you can. Send her a cute or funny card once or twice a week so that she gets mail. Send her pictures of her "sweet - mostly" (I love that) grandchildren often. If it's feasible, send an audio CD you've made of your kids singing, laughing, telling Grandma they love her. Sending flowers once in a while is expensive, but if you can, do it. Let her know that age and illness doesn't diminish her value or your love for her.
I'm sorry if I sound preachy, but I've been through this several times with relatives, and I know how rampant depression is in ill and/or older people. They wonder if life will ever be good any more. The rest of us need to answer that question with a resounding yes.