Why do i have this damn feeling that this will be situation like :
Lalya - How could you do this to me El?! And you MC i thought you like (love) me..!!
Elena - I'm sorry...
Layla - I thought you were my sister. I gave you all my old stuff...
MC - It just happend. It's all my fault...
Something in those lines.
The messy break ups thus far have been somewhat lacking in "fireworks", the "completely in love Layla" takes super well and is super mature and friendly after being ditched for her sister for example.
If you push things too far they also find out off-screen... and deliver a 2 minute cussing, meh.
A little exception if Erynn catches you banging Alexis after you almost went the whole way with her, that one has tears, lies and slaps to the face lol.
So Santi is MC's dad and Gabi's bloke and Gabi's no longer with him. Mr Monroe (assuming that Ana took his last name and didn't keep her maiden name for professional reasons) is L&L's dad and no relation to MC. Thus no incest for all these 'cest fan hangers on & my thought that they could be actual roommates in college or just friends from way back when could be true.
As for not speaking Spanish in their house, my grandmother had a very Spanish last name and couldn't speak a word of it. Not all families celebrate their culture. Speaking of that the Monroe name could just be an anglicized version of his last name, similar to how James Roday changed his name or he could have pulled a Martin Sheen and completely changed it...
I'm sure they'll know each other in some way... that's just how things work here

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Adriana, Layla and Alexis go by "Monroe", i think off-spring tend to go by the father's name, so it should be the guy's name.
All the "evidence" i gathered that their family isn't latina can be ignored of course, it's not solid proof at all, i'm just speculating on why i think it wouldn't fit.
Santiago is a latino first name, so whomever Santi turns out to be... that family plays/played we are latinos and proud card.
Maybe his mom is Latina but with a Murican dad, it's certainly a possibility(ugly mismatched name though).
Btw the Sheen's are named Esteves on their drivers licenses, Sheen is a stage name.
Since we don't see a reason for Monroe to need a stage name...
Also for Mr Monroe to be Santiago, means he hit Adriana to produce Alexis, then hit Adriana and Emily pretty much back to back to create the MC and Layla... the ultimate playa, i wanna play this prequel!!!
But i'm still putting my bets on Gabi's cucked husband.