I think that siblings of the same sex are a little bit more competitive when they are younger. I think they fight and argue more. But then, as they get older, they form a very strong positive relationship. I don't know if this is something scientifically proven, it is only my observation of friends and family :)
My sister and I argued and fought a lot when we were children, but now we have a great relationship. And maybe we do resemble each other more :)