Yep. Saw an interview with Kate and Oliver Hudson recently where they talk about their dad, Kurt Russell. He’s not their biological dad; he never married their mom, Goldie Hawn, even though they’ve been together for more than 40 years; he never legally adopted them. But he raised them, so he’s their dad. They call him Dad. Or, of course, Kurt.
And if at any point either of them individually want to fuck Kurt, and he’s down to pound, they can. It’d be horrifying and offensive, but it’d be totally legal, and totally not incest. (Sure feel like it though.) Woody Allen and Soon-Yi Previn were in the very same situation, and even got married.
That can easily be Santiago’s relationship with Alexis and Layla, and solves the incest problem.