Long story short, MC had sex with succubi queen when he was a boy, she came back 3 years lata and gave the kid to him saying it was his.
Years later, princess (queen daughter) meets MC and makes him a member of her harem (she is a kid, and needs his "mana" once she becomes of age in their world, which is pretty soonish
The queen still has a thing for the MC btw, calls him darling all the time and flirts with him.
So yes, the main romance is about a daughter and a dude that had a baby with her mom lol.