After replaying some old save on Layla's route, I stumbled on a scene at the end of chapter 11 between Santiago (possible biological father of mc / Alexis+Layla) and Adriana.
Adriana is talking about having another child and she says: "We always talked about having 3"
An odd sentence to say, if you aren't talking to the father of your two other sons.
As for the incest being forbidden, well... you must be aware of it. I remember a case from a few years ago about a young married couple, who's been together since the school, who discovered that they were brother and sister only because they were experiencing difficulties in having babies and started doing some genetic tests to look for potential causes of infertility. It turned out the (deceased) mother of one them had a liaison with the father of the other and didn't realize she got pregnant from him.
As for Gabi being married to Santiago, I believe it's just a case of re-using a similar asset for the picture. Unless I'm mistaken, we never get to see the husband in person; for whatever reason he left and he just speaks to her through phone.