In these novels, there is a lot of inbreeding because cats follow laws called the Warrior Code which states they cannot have mates outside their Clan, and therefore they do not have many options for mates to choose from, and most pairings in the books are at least distant cousins. Spottedleaf's Heart: this novel is entirely about an apprentice (about 12-13 if she were human), being groomed by a middle-aged man (if he were an adult). It does not show their relationship in a bad light, and the teenager only stops contact with the adult after finding out that he trains in the Dark Forest (what is essentially cat hell in these books.)