Well even if Santi would be the stepfather of Alexis and blood father of MC, Alexis and MC have no blood or legal relation as such or ever been part of the same family living as a family unit. So none of the protective laws would apply or definitions of incest however broad you want to make them. It may be intertwined though.
![Stick out tongue :p :p](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Nikki and Cara would be a bigger danger of that, though considering the age they first meet, that would be really stretching the idea behind those protective laws.
Even if Santi would be her real father as long as he was not mentioned on the birth certificate of MC, no outsider would have to know as long as the few people who would know kept their mouth shut.