Personally I probably wouldn't move in with the aunt to begin with. But suppose I would, I think in most of such cases there is some mutual benefit. Perhaps in exchange for moving in, you are expected to have tea with the lonely aunt at least twice a week, and to tend to her garden (without that having been agreed upon beforehand). In any case, it is just not nice to promise something, make other people invest in your promise, and then suddenly throw out your promise. Whether it is good style by the other party to accept something for free doesn't even enter the equation at this point.