I've just noticed like most of you have, that most of the crews parent's aren't really mentioned much.
Usopp: Mom died, Dad's a pirate
Nami: Parents died in pirate invasion
Zoro: Not mentioned
Luffy: Not mentioned except for his grampa(Gold D. Roger?)
Sanji: Not mentioned
Robin: Parents died in buster call I believe. I'm really not sure correct if wrong.
Chopper: shunned by his parents.