Hmmm, we [old people
] get real stubborn in our old age and, although I can understand why the grandmother doesn't want to give her up yet, it's a shame that she feels that way because she is not benefitting the bird.
It seems to me that you don't really have much choice... You could make a huge effort and go there once a week to spend the day with both the grandmother and the macaw - it might change the grandmother's mind when she sees how committed you are to the bird's happiness and how you are willing to sacrifice for her AND it will start the process of her becoming used to you.
But, if you can't, then you will have to wait until the grandmother gives her to you and hope for the best. When that happens, there are things you can do to minimize the stress. For example, one thing I would do is to arrange for me not to have anything to do outside the house for a number of days and to have a nice room ready for her. People tend to underestimate these birds intelligence and their capacity for adaptation... All my birds came from somewhere and somebody else and they all did VERY well. Parrots are smart and they know when they are being treated right and she won't be the first bird to go from one owner to another [it's estimated that large parrots go through 7 owners in their lifetime] so, if she is not plucking now and her conditions in your home are better than they were before (and I am sure they will be because an old, sick person cannot take good care of a macaw!], she will adapt very fast and quite happily. So, don't worry too much about it. Cross that bridge when you get to it and, in the meantime, I would suggest you volunteer at a rescue where they have macaws so you can start learning their body language and how to handle them. And, whenever you have the opportunity, go visit her because even if you only manage to do it a few times, she will remember you and that will work in your and her favor.