I know there are many threads on this - I don't have time to search right now - but IIRC many programs will allow the points of deceased spouse to be transferred to your account. I'm not sure if this extends to siblings, offspring, etc.

If I had a relative who passed and would have been cool with me doing so (and no other family members entitled cared), I'd probably go the route of just logging in and redeeming as if they were still alive. The odds of being caught would be slim. For any large accounts, I'd consider converting the affinity points to gift cards or similar items that would enable the asset to be secured apart from their name. For hotel points or other redemptions that might pose a problem using without the person alive, I'd consider converting the points to airline miles via various methods, or again, gift cards and the like.