75yr age span from grandma to son, and I take care of both. How long can I do this and keep my health?

Take breaks. Caretakers can wear out & become ill themselves if they fail to arrange periodic breaks from caretaking. Other relatives may spell you or you can arrange for trusted professionals/friends to give you a day or two away from the stress. Council on aging may suggest daycare center for grandma, for example. If your son has special needs, you may find a nearby support group that can help.