Lots of good ideas here - just sending good wishes.
One other suggestion - have a regular time when someone looks after ill sister and he has a day of personal attention from one parent - and this happens regularly so he can come to rely on it.
One of my DDs starting soiling herself when she was about 8 and we dealt with it by upping the attention (not at the time of the soiling of course, just in general) and it went away - I think she was trying to tell us something!
Have any of you got all electric cars? Pros and cons please.