family business
Family business or "the family business" is when two or more related characters are present in a romantic or intimate exchange, but not with each other. Rather the intimacy or romance is pointed to an unrelated third party. This could be a love triangle or harem involving a mother and daughter interested in the same common character, at the exclusion of that character also being in the family. With intimacy this could be with a threesome where either both members are independently having sex with the third party, or the family members are assisting one another in aiming their attention to the third party in a way which does not involve penetration or placing affection on each other. Affection in this sense being the sort like a passionate kiss, cuddling, or other non-intimate acts which could still be notably incestuous.