Aside from two friends, one happened to login while at the other's house etc, I never saw an instance on DDO of two siblings, close relatives or anything being on the website. Never, not once even.
I believe it is in fact an unventured-into aspect of sibling development as well as even further out into a family that results in only one of a particular group generally being genuinely argumentative and passionately into debating.
You'll easily counter me here with 'toxic families' or instances where many butt heads but what you'll tend to find is that those less adept at the use of words, develop habits as they grow in adolescence through to adulthood that result in them less needing to argue directly and they defeat superior debaters by these social or physical means (you can't 'beat people up' when you're a grown adult but you sure can out-work them and point it out to shut up whatever possibly smart idea they had for the workplace etc).
I think that there is almost a rule, not a trend but an actual somehow rule, that as siblings or whoever (could be cousins or friends if they interacted a lot) develop, the ones who discover they're not as initially gifted at arguing with the other/s learn to avoid argumentation and seek reconciliation or other ways around debating, as a habit (not even innately gifted, just initially, meaning younger siblings in general have lower probability to wind up being into debating).
This 'habit training' also works in reverse. The more one realised they had an edge at argumentation, the more genuinely they worked on their craft and became superior expressors of opinions and persuasion (or at least argumentation). This side of things is probably truer for males than females though because the females gifted at arguing tended to need to be skilled socially regardless (just how things are) whereas the males skilled at it, especially in the modern age of the Internet, could find outlets where they were not really held back by their social awkwardness or physical weakness (I am no exception to this rule and not ashamed in admitting it).
Eventually, either the person becomes a healthier and more disciplined debater and human being or they alternatively get more toxic and argumentative as they grow (the hostile hermit elder stereotype).
