I think all parents want their offspring to be each other's best friend, yet parents need to face the fact that siblings can be, as you note, "too different." Their personalities may clash; their interests may be too diverse for them to be close in the way parents hope.

The research shows quite clearly that the bonds between adult sisters are closer than those between brothers or brothers and sisters. Not knowing the ages of your sons, I offer this: Siblings tend to get closer as they age across all the sibling combinations.

You watched carefully...didn't allow their differences to escalate into sibling abuse which would have prevented them from being "friendly now."