Despite only getting back into the Iconoclasts fandom after so long, one personal headcanon still persists in my mind...that Mina - a pirate girl from another culture - acts as more of an authentic older sister figure to Robin than Elro - who is Robin's actual biological brother. And I'm sure there are other people who think that way as well.
But in all honesty, Elro...isn't actually a "real" brother. At least, not to me, in a sense. Not in a way that matters, anyway.
As far as relationships go, Iconoclasts does its best at showing off relationship dynamics, and how they affect other people. Grey went out of their way to court Black for a whopping 187 years, breaking down her barriers with utmost love, care and patience (up until Elro killed them, that is, and that's something I will never forgive him for). Royal met someone who also held an appreciation for flowers, which he loves the most (especially since Robin's nickname is "Sunflower"). Mina decides to stop running away, and turns over a new leaf to become a better person and secure a future that she can share with Samba.
The same can be held true for familial relationships, especially in instances where blood doesn't always make a family or even siblings. It's especially true when you compare how Elro acts around Robin to the way Mina treats Robin, or even how Mother treats Royal, her adopted "son".
With Robin and Elro, there is little warmth in how they interact, especially after the deaths of Henetta and Ella. I'd argue that there is very little love between them, either. Elro claims that he cares about Robin, but that "care" is relegated to how she should stay home where he thinks she'll be safe, even despite the fact that she's practically an outlaw and staying home will result in guaranteed Penance for her. Before he died, Polro wasn't much better off, either, insisting that Robin live a safer life as a farmer, like her mother and maternal grandfather. Robin literally finds more comfort in the machines she makes, and that's heartbreaking! And it is only after Robin kills God himself and restores the planet that Elro finally gives in and tells her that he won't tell her what to do anymore.
Robin and Elro may be siblings, but Elro sure as hell doesn't present himself as a kind and loving brother in a way that all older siblings are meant to. At least, not after Polro's death. Even Black sees Elro as a piece of garbage, though that's only due to him being responsible for the death of the first person she came to love.
On the other end of the spectrum, you have Mina. Robin just met Mina, and the two don't know very much about each other. And they only end up getting together when the One Concern attacks Isilugar, and then when Black kidnaps Samba after White dies.
And yet, within the short timespan...you can tell that Mina and Robin's relationship is far more genuine compared to that between Robin and Elro. There is love, care, respect, and protectiveness. Mina encourages Robin to go out and make her mark on the world, instead of sitting at home doing nothing. Like Elro, Mina worries for Robin's well-being; however, unlike Elro, Mina also acknowledges that Robin is her own person, and that she perfectly capable of handling herself, especially after Robin solos Black in the detritus sand pit. They may not share the same blood, but the feelings they exhibit are so real despite Mina not necessarily having the purest intentions at heart compared to Robin. Not to mention the way they work together during boss fights, with Robin incapacitating the opponent and leaving them wide open for Mina to make the killing blow (the Inti boss fight is a perfect example of their teamwork, especially when they high-five one another). Mina is able to establish a warm place that Robin can call home, and acts like a "real" older sister.
I might be looking a little too deep into this, given that I'm a huge sucker for the "found family" trope, and I like seeing how two unrelated people can develop familial or sibling bonds compared to "official" or "real" families/siblings. Kinda makes me want to write a thesis paper about it, lol.