I have a brother, and whilst we get on absolutely fine, in reality I'm very much a girl's girl (love shopping, spa days and the like) whilst my brother is a typical bloke. He loves his sport. It means that we have zero common interests. We've never not got on, but I wouldn't say that we have loads in common.
But, I have two daughters who are as thick as thieves. At 14 and 17, they're already planning their first holiday away together alone. They have their own friends, but still share a combined hobby, have regular sleepovers together (even at their age!) and enjoy socialising together. I have no doubt that they'll be very close as adults.
But here's the thing. For every advantage there's a corresponding disadvantage. I'm not hugely close to my brother, but I love the spa days and days out with my mum. I don't have a sister to do it with, so I do all that stuff with my mum. Whereas, my girls have each other. As I've said, they've already planned their first holiday away together, and already do days out shopping together. I'm very much the spare wheel in their relationship. In some ways, it is kind is sad that I won't have that relationship with my children, that I have with my mum. My girls have each other, they don't need or want me much of the time. So I'm very aware that every advantage all too often, there's a corresponding disadvantage.