Ok then time to drop a bomb ! mwahahahaha, just logical deduction,
- Santiago -
you and me too at the beginning, tought he is unfaithfulll and has many ladies etc etc, just like the clasical "Galán" (seductor, lady magnet) but, the truth is that, we don't have really proofs on it, in the flashback from Emily, he wanted to asume the responsbility of his son, this was avoided by the apearace of his parents,( which I asume he didn't know about) , and by Andrés, playing the "knight in shining armour" and gaslighting Emily about Santiago.
Adriana on the other hand don't strikes me as a woman who would tolerate any cheating, (of course the argument that she don't knows) but it is very difficult to hide such a behaviour through out years, moreso if they have a close relationship like they have.
And the one we have proof is Andrés, he is a cheater with narcisitic personality, and resentfull.
My take is that Gabi (we know that Santigo was her agent) had a crush on him ever since, and Ximena knows and teases her as good (sister?) or bestie