Feeling overwhelmed: husbands death 4 yrs ago, Dad's 2 yrs ago and now Mom's (84) decline. Any advice?

I agree with what Pam wrote above. If you were a full-time Caregiver for your husband and for your father [so sorry to read about both of their passing] you already know how difficult and exhausting this would be.

If you weren't a Caregiver to husband/father [depending on how they passed], then I suggest you spend a few minutes reading under the discussion section "My whine moment today. What's yours?" and read about all the issues that do come up.

Your daughter is your first priority . Nana called this "rota" meaning to turn. She took care of her daughter, who raised her own daughter who raised her own daughter. It's your turn to raise your own daughter. Cancel the sale. Get mom to Assisted Living, because old people really would rather play with people their own age. Sell her house to help pay for it.
Moving in with Mom does not work 99% of the time. Ask those who tried.