I'm sure in many ways CLKD is right in what she says, but I know it's incredibly difficult for many of us to be so (what seems) clinical and detached from our families, especially when we are close to them.
Re your Mum - does she have a social worker, or a contact number for her Local Authority Adult Social Care Team? Talk to them about what help might be available - although I fully understand you may want to be involved in your Mum's care and help her where you can, you will be no good to her if you make yourself ill trying to do too much. Be realistic with yourself about what you can do, and firm with Social Services - there is help available.
Re your father-in-law - what an awful situation for the whole family. Whatever you do, don't feel you should 'be strong' for others all the time - you need to find an outlet for your grief and anger (especially if you feel you can't share these feelings with your husband at the moment). Has your FIL been referred to a hospice? If so, there will be support available through them for family (and friends for that matter). If not, give MacMillan a try. You have every right to feel whatever you are feeling about your FIL's illness and what it's doing to you, your husband, your family, and you need to find a way of expressing your feelings and working through what is a horrible time.
Re daughter - on this one, I'm much closer to being 'with' CLKD! It is terribly sad to see our children hurting, but all we can do is offer support - we can't 'mend' it. Have you suggested she talks to Relate (either alone, or with her partner if he's willing). They'll be able to offer help in a way you simply can't as you're too close. Be there for her if you can, but not at the expense of your own health.
And after all that, please make some time to spend with your husband. He's going through all this too and no doubt needs you as much as you need him, but he's a man and chances are he won't show it, or tell you! You don't have to force heart-to-hearts - just make time for a walk in the countryside, a pub meal, trip to the seaside - whatever the two of you would enjoy together. And for those few brief hours, turn the phone off and let the rest of the world go to h*ll.
Please - find someone to talk to xxx