Jacqueline, I couldn't believe that, in addition to your other suggestions, you didn't suggest that the married sister should either travel the 1000 miles and buy her single sister a well-deserved holiday or else PAY for someone to care for the mother while the single sister went on a holiday paid for by the married sister. Surely at some time in the six years, the married sister had taken holidays. Did she ever think of taking her single sister's place during some of those holidays??
And how patronizing to suggest that the married sister ask her single sister for advice "even if you don't really need it".
My advice to the happily married sister running a successful business would be to get off her butt and give her single sister some relief!!! If she truly can't be there and since she runs such a successful business, then she can afford to hire someone to relieve her single sister. Do something practical about feeling "so guilty".
Furthermore, she's had two extensive trips out west to see her daughter, while I've done everything else. I've asked my sister to let Mom come "for a visit." She tells me, "I have a headache that day."
Right now, she's attending a funeral for her deceased 21 year old daughter in another state. She said I can bring her to the second funeral/memorial service (9 hours away), but doesn't want Mom in her house. She can't she "can't deal with her drama." So...guess what? I won't be taking mom to the funeral at all. We grieve the loss, but also sis' relationship with mom (and ourselves). Sometimes people can just be self-serving and inconsiderate.