I don't know if there's any accepted etiquette but it seems like you'd first call parents (either set) then siblings and then other relatives and friends. If you have kids who are waiting at home, I'd call them first (especially if they know you've gone to the hospital).
If you call one set of parents first this time, call the other set next time. Or, tell first the parents to whom you announced the pregnancy last... Or take 2 cell phones and call simultaneously :-)
As long as mom & baby are healthy, I don't think it makes too much of a difference :-)
Good luck!
Rachel (mom of 5)