I just want to add that growing up I was extremely close to my siblings.
I have a sister who is 16 months younger than me. She and I grew up as close as twins and remain that way to this very day. My brother is five years younger than I am and four years younger than my sister...and then our little tag-along, my youngest sister, was 11 years younger than me. We did absolutely everything together growing up, the whole pack of us, and while I can think of a few pretty nasty brawls we had, most of our disagreements were quietly and calmly worked out.....for kids, we had a very good sense of fairness and, often times, the two who were in disagreement about something would each tell their side of the story to a third sibling and that third sibling would say what they thought was fair. We very very rarely bickered. Here's why:
Our mother did not tolerate fighting. Period. "Fighting children are children with not enough to do" she used to say - kids caught bickering were put to work. Immediately. Polishing silver, folding laundry, mowing the lawn, you name it. If there wasn't any ACTUAL work to do, she made some up. "Move that pile of rocks from there, to over there...and then move them back again.
We never had anyone making up fun games for us. We never had anyone separating us because we were fighting and my mother had ears that could hear a mean whisper....if she heard anything that even SEEMED like it MIGHT be fighting..."You, you, weed the garden....YOU take the baby and go see about the laundry"...every time.
Fun was up to us. Fighting was not tolerated. Everything we did, whether work or play, we did together. Team work was key, avoiding the slave driver (my mother!) was critical.
The best game we ever played was "steal enough apples, bread and peanut butter to keep us fed for the day, pack the baby sister in the wagon and get out of the house before she could find ways to make us busy.
To this day, the four of us are unbelievably close. We speak on the phone constantly and are here to support each other now that we're all forming families of our own...it's beautiful. Better than I ever could have imagined.
When I was a kid, I thought my mom was a mean mom. In many ways, I still think she was harsh. But some of the mean things she did, I can see now with the clarity of an adult, and I realize that she was trying to build us up as a team. She was trying to instill a sense in us, that all we had that we could truly count on, was each other. We have made it through some rough times together and my siblings always have my back through thick and thin.
In everything I do with my kids, I support attachment to me, to their father....but the relationship I spend the most time thinking about, is the one between the two of them. That's why my DD has chores and my DS, since he was old enough to walk, tags along to help. Even if it's holding the bucket, holding the hose. When they get a snack to take outside, they eat from the same bowl...always have, so they don't even think about it. When they get a piece of something to share, I never break it in half for them...I hand it to them and say "go on now, share it" - how they break it up is up to them...and don't think for a second my DS doesn't pay attention. When they nap, they nap together. Everything together, always.
The reward for this, is that at the end of a long day of cooking together, napping together, sharing plates, working on DDs chores together, etc....I get to watch them hold hands and run off into the woods together, where they play and make up songs and stay away from the house as long as they can.
I'm not saying I have anything figured out....but there is a science, I believe, to the art of relationship building when it comes to siblings. I know people who have kids who are at each others throats and I know people who have kids who work, play and problem solve very well together with minimal fighting. I think personality comes into play, but I also think that the way a family is structured lends itself to closer or less close relationship between siblings.