When my mother could no longer care for herself properly she moved to our state from Florida. She was able to rent a lovely small apartment within site distance of our home. [Our house would not have been a suitable long term residence for her for various reasons.] This apartment would have been ideal if she had had enough memory left to participate in community activities.
By the time she moved to our town her Alzheimer's embarrassed her because she could not remember names of people, and so forth. She mainly stayed at her apartment with her cat, or we brought her over for visits. This was a very difficult time for all of us, because our children were becoming adults during the same years, and we were torn in two directions. My DH and I also both had hectic jobs, too.
We finally had to put her into a nursing home on a closed wing, after she called the local police and reported her hallucinations as real. That was a very sad time. I was shocked though, to realize that I was so very relieved not to have to deal with her care any more. I hadn't realized how much time I had been spending cooking for her, taking her to appointments, sorting her pills, etc. Just knowing that mom was safe, and that she was getting good meals and proper medications lifted a burden that I hadn't realized that I had.
I would advise against moving to an isolated area and being your mother's caretaker. It is an incredibly hard job, and unfortunately is all down hill. I think that you can tell your dad that you will always watch over her, and make sure that she is OK. I think that your dad is concerned about your mom, but doesn't understand how difficult this big disruption could be for you.
Your DD's still need your love and attention, too, even though they are young adults. If you became your mother's caretaker the duties might detract from the attention you would want to give to your girls. In a few more years there may be grandchildren to love and cherish, also.
I bet that your mother is a very practical person, and has thought of different possibilities that could happen. I think that you should ask her what she would like to do, if dad passes away and she finds the present home to be too much for her. Maybe there are assisted living places in your area, where your mother would have some independence with a little apartment, but still be near to you.