the suffix -ship in English always implies a certain quality associated with the word it connects.
for instance:
friend and friendship: the former indicates a role of a person as in "he is a friend of mine". the latter emphasizes on the feeling of friendly affection. in real life we may well have many friends yet only a few can offer friendship.
craftsman and craftsmanship: one may be a craftsman but not necessarily possesses craftsmanship.
leader and leadership: you can see too many leaders who have no leadership at all, which includes vision, courage, charisma and spiritual quality.
i think you have got the picture.
a relation may or may not have the relationship, so to speak. the two words have their own emphasises.