I have a throuple who have kids together. The ones who are adults have moved out and gotten married but they come over and flirt with their adoptive caretaker. I have 2 sims with 8 kids together and ...
Ideas
There’s 12 sim kids in total
so if sim 1 and 2 have a child and sim 3 is the adoptive caretaker, sims 3 flirts with the adult sim child of 1&2. Same if sim 1&3 have a child that sim 2 is the adoptive caretaker, as soon as that child is an adult then sim 2 flirts or kisses them. And same if sim 2&3 have children, when they are adults they start flirting with their adoptive caretaker
Since they aren't related, they are allowed to do that.
Make sure you don't have the free love NAP active if you own the eco pack.