Since their sister ships does this count as incest to some degree?
That would require knowing a lot more about how shipgirls are actually "made" to answer. It's never entirely clear whether shipgirls are humans with the "ghosts" of ships inhabiting their bodies (in which case the ghosts are "sisters", but the humans are likely not related, except maybe in the case of "twins" like Ikazuchi/Inazuma or Hitomi/Iyo), or they're some sort of robots (in which case they have no genetics or blood relations at all).