Helen, I've given this some thought and I'm sure I've said similar to you before but nevertheless, here it is again.
Your mother in law is not your responsibility. You don't get on and she causes you a lot of stress as well as takes up a lot of your time.
Write to your brother-in-law telling him that in light of your new job, your husband's poor health and the dreadful relationship you have with M-i-L you are no longer able to deal with her problems nor her situation and in future will email any issues that come to light to him for him to deal with.
When she phones you, be sympathetic but tell her you'll let B-i-L know and keep the conversation as short as possible. Then email your B-i-L for him to deal with as he sees fit.
Turn off your worry button.
If she phones for help with Sky or any other problem, tell her to phone B-i-L.
Really, what she does with her money is her concern and if she wastes the lot on insurance she doesn't need, so be it.
Stick to your guns.