Every parent has some script surrounding the close, lifelong relationships that they'd like their children to have. Maybe you have one - my sister has two friends that she's known since she was three and they are still close more than 30 years later. I have one friend I've known from birth.
There's a familiarity and comfort in long term friendships. Tracey's friend Caroline calls my sister "the witness to my life". There's not a lot to explain to these long-termers...they were probably there when whatever it is happened.
I hope that my children are each other's close friends as adults. And I'm stacking the deck to encourage my kids to have another set of lifelong friends - their cousins and their almost siblings. Dr. Jessica's husband took Taylor at mid-day Saturday and kept him all day - giving Taylor and Ilan a 8 hour playdate plus sleepover. Their friendship doesn't resemble school friendships - but it doesn't really resemble siblings. It was special that they got a day together. They participated in a lot of different activities. When one got upset, the other knew how to comfort him. And when their older and younger siblings joined the fun at 6pm, they adjusted and starting playing with the other children seamlessly.
Almost siblings don't live together and don't get to see each other every day. But they are a definitive and reliable part of life and see each other frequently. In a few years, they'll be each other's telephone buddies too. Interestingly, these almost siblings seem to be the safety net for the kids - the friend you KNOW you have even if you don't know how to fit in at school or camp.
Does your child have an almost sibling? How do you encourage this relationship? If your child is older, did the relationship last?