Sin, Unending Cataclysm

If the target creature is an illegal target as Sin's last ability tries to resolve, it won't resolve and none of its effects will happen. Sin will remain in the zone it went to when it died (which is usually the graveyard).

In some unusual cases, you may end up putting the appropriate counters on more than one permanent. For example, if you control The Ozolith ("Whenever a creature you control leaves the battlefield, if it had counters on it, put those counters on The Ozolith.") when Sin's last ability resolves, you'll put the appropriate number of each kind of counter onto both The Ozolith and the target creature.

Sin's last ability puts all counters that were on Sin onto the target creature, not just its +1/+1 counters.

Sin's last ability doesn't cause you to move counters from Sin onto the target creature. Rather, you put the same number of each kind of counter Sin had when it died onto the target creature.