I agree with Imperfect - look for help. This is what I would do, obviously take or leave as is most helpful:
Your DiL should have a specialist team, including someone like a Macmillan Nurse - ask to speak to them, on your own (+ your DH if best). Normally they will do that happily. Arrangements for support do vary from area to area, so begin by asking your SiL, and if you get nowhere, then contact one of the charities, and ask what support is available in your area. Similarly, there should be a child bereavement team who can offer you specialist advice: this is the one I know best:
www.chums.uk.com/ but I know of Winston's Wish as well.
Whilst the family need to support each other, you & your DH need support away from them, including knowing how best to handle everyone. Any of us find the caring you are doing tiring, but the emotional burden adds to it.
You are going to be doing a lot more of this, so think carefully about your priorities. This is what my list would look like, yours may be different:
1. get a cleaner, so some of the everyday stuff is less of a burden
2. get a freezer full of ready meals, at the very least, some pizzas and ice cream
If either of these is difficult to afford, consider asking whichever Cancer charity is supporting you - they are flexible and helpful.
3. Who else helps, and how best to use them? Obviously DiL's family, but there may be practical issues. Are there 'swimming' parents who might help - many friendly families are happy to do what they can (in a similar situation I took children to a youth group & activities to give a granny precious time). You can even consider a taxi for the actual 'run' if there are friends there to look after them / cheer them on.
4. Make a little sacrosanct time for yourself & DH so that you can both recharge your batteries and do your grieving.
I with you and your family th best you can hope for - some more precious time with DiL, and the best ending there can be.