I'm really upset that Edora didn't react to being considered only a "sister" after the headpat... her reaction was portrayed much cuter in the novel, where she was shaking in despair at the stands upon realizing she's only considered a "sister" by the MC.
That's basically it. Well, daughter or any woman for that matter. He can pick as many of them to be his wife, but he needs to sleep with one woman from each family to produce an heir.
@Saxfire Edora Does say Jang is their half-brother. Plus you can see the family tree the frame just before with all the heirs connected to the same king.
@Saxfire because it states the strongest individual becomes the king, a new king is chosen from the previous king's children, to give each of the 51 tribes a fair chance he must have a child with each tribes since otherwise they aren't eligible to have a king come from their tribe for that generation.