Unless Gabi ADOPTED MC there is no incest. At all. Even if she married his father, it doesn't matter.I don't think it's incest but if it is categorized as incest in patreon well it is what it is.
"unless they were already divorced"
Hell, even step-sibling sex is not incest if there was no adoption involved by the new parents. Imagine you're 18 and your father marries a woman with a really hot adult daughter. You can fuck her or even marry her, it's legal because since you're adult there was no adoption involved. Hell, you don't even have to be adult - after all what would be the point of adopting a 16 year old when you marry his/her mother or father? It's just a needless complication, plus if you divorce the parent you have even more problems (it's little known that more times you marry the less chances you have of lasting - 50% of first marriage ends in divorce, of second marriage 67% and of third marriage 73%). They are practically almost an adult, they're not a child and don't need such supervision and you don't need the legal powers of adoption to take care of them and will be out of the house in 2 years. So, even two un-adopted not-yet-18 step-siblings should legally be allowed to have a relationship.
That doesn't mean you won't get flak from people around you, but legally it's not an issue.