Given that more than one family - and more than one child - is involved, one other thing to bring up at this point in time is her healthcare directives and durable power of attorney: does each widow have the above in place? If not, you can buy complete kits on-line for maybe $100 that will get all of that filled out and taken care of.
If they both have trustees, etc., then perhaps let the trustee take the charge of initiating this discussion and issue (if it's not you/DW
).
Since the other siblings are moved out and away, have either mother ever made comments about "oh, I never see so-and-so"? Or are they in the same state of her grace as you and DW, who have so faithfully stood by their sides? I'd imagine that if/when you do leave town, their attitudes may change if you spend any appreciable time with her currently, and they are suddenly left with no one (just as would happen with my grandmother - as long as I take charge and sacrifice myself, all is well...but if I moved out of town, hellfire and brimstone would raineth down upon everyone's head).
In the course of your discussion, I'd at least make the offer to see if either mother would be interested in looking into condos/apartments/facilities near the place you would be living - that way, in a sense, it is HER decision on whether she is content being left alone, or if being near a child is that important to her, rather than it being YOUR decision to 'abandon' them.
If one of them doesn't want to move, ask her what is keeping her there, since they apparently don't get out much - so the inside of a house in one part of the country shouldn't be that different than the inside of any other house/condo in another part of the country!