Here's the thing though: an underdeveloped character can "develop" characterization through their inaction. Here's what we know about Nikos:
He's the type of person that will "propose" to his niece's husband and then pulls shit like this:
He's the type of person that decides to let Marina and Andrew fight, rather than offer any kind of solution.
You're telling me that while they're hosting Joel, Nikos couldn't have made the world's easiest excuse, which is that Elena has taken her beloved niece Marina around Greece (or even generally visiting sites in Europe) over the next few months? Or even if he didn't think of it immediately, why is Nikos, who is seeing his niece so upset, just going "well, sucks for you. Time to get your husband femmed up so we can kiss and fuck some more!"?
When "Elena" is suddenly extremely distressed and asking about "Andrew", he's the kind of man that just "stays in character" and gaslights her?
When any rational person would take this as a hint that something is wrong, and say something like "Oh honey, I'm not sure what you're talking about", and then lean in and whisper "we should find somewhere more private to speak". Like, I take great issue with How Marina didn't do something like that, but at least she immediately tried to use the safe word and then was upset that Andrew wasn't responding.
Nikos is the type of person that makes out with Andrew and then wraps his arms around her in FRONT of her wife? And then proudly openly says that they're getting married? And then has Marina take part in this farce by toasting the "happy couple"?
All the while with that stupid smile on his face, and never even stopping once to check in on how Marina is holding up?
Nikos hasn't "done" many actions towards Marina, and hasn't technically treated Elena "poorly". But there's many actions he should have taken but didn't. He never tries to help keep Marina and Andrew together, he never tries to minimize the weirdness of the current events, never tries anything to keep Andrew from becoming more feminized. He never shows even the slightest hesitation. The closest thing he does was way back at the first dinner.
The best he could come up with to derail this train all the way back then was a shrug. And since then, he has never stopped once when it comes to getting physical with his "wife".
All of that characterizes him as a selfish, manipulative, bastard, and possibly disconnected from reality. This is sadistic behavior. Given his wealth and presumed intelligence behind his success, you're telling me that once Joel showed up, he couldn't have come up with the story: "My wife's been driving me crazy so I sent her away. Bitches, am I right?" Honestly, that would have probably played perfectly into the old man's "conservative, religious zealot" biases; Gotta put women in their place.
He's been on screen enough, been part of events enough, and the story has gone on long enough that his lack of action speaks volumes about the type of person he is.