Imagine what we could learn from them. It would be like the competition between Apple and Microsoft -- something that makes each company better. As long as it doesn't kill or maim, competition can motivate and teach.
But there is no sibling race with which humans can compare themselves. We had a shot with the Neanderthals but they didn't make it. That's probably a terrible setback for our species. We missed out.
Sure we'd wage war with the other species in our primitive years but at some point we'd reach detente -- if we didn't wipe each other out. And perhaps a sharing of values would take place in the period after that. It's possible, anyway. It would work best if the other race was both smarter and less warlike, compared to humans. Sort of a race of super-intelligent bonobos.
Alas, we have only ourselves for reference. There's little possibility of humans ever meeting another intelligent alien race. They're just too far away -- if they're there at all (which I believe they are, and in abundance).
Even if mankind lasts for a few million years, it's doubtful we'd be able to traverse the tremendous distances between Earth and the nearest intelligent aliens. It seems we're not destined to meet these creatures.
Kids with siblings are healthier and more emotionally balanced than children from single-child families. Poor humans, we're living in isolation -- and paying a high price for it. It really is a pity.