I doubt LW wants her niece to take over care.  However, my guess is that she refuses to come spend time with granddad, and mom won't make her, nor has she taught her the value of sacrificing time for yourself to honor elderly infirm relatives.  50 years from now we'll be seeing a letter from a woman whose daughter is always too busy with her own life to come visit her in the home, and never brings the grandchildren by.