So sorry you and your dh are going through this op 
This website might be of help. It has been put together by the RC church but contains useful info for everyone.
First (sorry if this sounds too obvious) it's best to check how much your mil knows about her own illness and prognosis. If you can be present, and having been through it with two sets of parents, I would say, tread very lightly, don't go rushing in (however well intentioned) just sit alongside and be there and ask open-ended questions such as "how are you feeling" or "is there anything I can do for you or is there anything you would like to talk about?" and take your cue from her as to how much you feel it is appropriate to say.
It's not maudlin at all. Death is part of life 
If you can't be there, maybe a card might be a good way of communicating what you want to say; again, checking first with your dh what he thinks is appropriate. I say this because my late mother was very vocal and specific and discussed death openly. My father though, knew he was dying, and we knew he knew, but he didn't want it discussed openly. Everyone copes differently.
Ways of helping your dh could include listening, picking up the practical slack at home while he is away, doing his chores and and lifting the load a bit, and maybe getting the DC on side so they don't demand too much of him ATM, and of course, feeding him well. Maybe making a Dundee or ginger cake or frozen dishes he and his brothers can eat when they have a moment. If you can be present, offering to sit with your mil while they take a break? You are already helping by keeping the home fires burning.