Did you go through the place with them before they moved in and list any known problems? If not then most likely a judge would throw out your claim if they took you to court. If you did and the cracked sink wasn't listed then charge them for it.
If it wasn't on the list then charge them. They had plenty of time to call you and say hey what's this crack in the sink. Someone was most likely careless and dropped something heavy on it and they don't want to own up to it. I wouldn't say anything more to them about it and just put it on your finalized statement to them.