The "parasite" in your story is actually the grandmother. Realizing that the system having "parasites" is not avoidable and is not a moral failing is huge.
Ignoring for the moment the argument that the grandmother may well have paid a load of taxes before getting alzheimers, one thing that would seem fundamental is that the grandmother gave birth to a parent of the carer, without which the carer would not have existed at all, so how can the grandmother be considered a parasite?
"Parasite" is kind of a charged word. Trying to reinvent it as something OK is walking blithely into doublespeak territory. If you feel you need to redeem it, then okay, but I'm not seeing your reasons for wanting to.
