We spend a lot of effort trying to help the kids learn to be kind and encouraging them to work together. We do a lot of things together as a family and encourage shared interests; my family comes with me on most business trips even though it would be far easier to go by myself (it gives the kids many opportunities to travel). Both kids are very empathetic in general. I can give you a long list of examples of considerate things they have done. For whatever reason, though, they just have trouble being friends. I don't think the fact that kids are not friends means their parents didn't teach them to be friends; some kids just have very different personalities and that can be challenging with two intense kids.
I'm sure parents can increase the chances that kids will be friends versus resenting each other and feeling unfairness, but they can't guarantee it.
