I had the same issue with my MIL starting when my FIL passed 23 years ago. Until then, she was very self-sufficient - paid all the bills, grocery shopped, tended the garden, etc. When he passed, all of a sudden she needed my hubby to pay her bills, take care of the banking, do all the yard work (2 acres worth) and this was when we had an infant and a toddler!!
My hubby has two older brothers and a younger sister. His older brother never did anything (right up till she passed two years ago) and his sister only ever "visited", though in Mom's last few years she did take her grocery shopping every now and then.
Other than that, it seemed that duties were divided between my hubby and his brother. As she got older and sick, they did everything for her - from moving her into assisted living, to taking day/night shifts with her the last week of her life. I never understood why she didn't ask the other two to help out, nor why my hubby and his brother didn't either.
I can tell you now that she isn't here anymore, putting up with the hubby taking some time to help her out is better than wishing we had done more. I still feel for you though!
__________________  Sissy & Angel |