My 7 y.o. DS and 4 y.o. DD play together constantly, they are best of friends. They both climb trees, they both play with dolls together, they build lego contraptions together, they read together, they do imaginative play with stuffed animals together, they play computer games together, sitting side-by-side on the same chair, they cuddle (or at least sit right next to eachother) on the couch while watching a t.v. show, they create entire highways together and set up all the Matchbox cars along them, etc. They shake hands and give eachother hugs and kisses when they apologize after arguing. My eldest will sometimes make breakfast (cereal or toast with jam) his sister if they get up before me and she always says, "Thank you very much, ___".
I truly don't believe that my children would be closer if they were the same sex.
I only have our family and my family growing up as my experience (naturally) but I believe it has to do with your family lifestyle. For instance, when I was very young, my brother and I were very close, but once we started going to school (out of the home), we each had our own friends and as we progressed through the grades, there was an age discrimination that we learned, little kids are boring, that type of thing, and I (being the eldest) was no longer very interested in playing with my little brother.
I do have relatives with twins (one boy, one girl) and they have always treated the children differently, even as one year olds, telling the boy not to cry like a girl, or not to play with a doll, "that's a girl toy". [It was just horrible to witness, thank goodness we never spend time with them anymore therefore our children do not need to witness it.] Certainly, in a family like that, I can see that a brother and sister would have fewer common interests and might not stay close for as long as they would otherwise. At the age of 8 these twins have only complaints about eachother, even when talking on the phone with grandparents.
In our family, I think that homeschooling has really helped with family togetherness and even though the children have plenty of friends outside of the family and we spend much time with them, they either play with their friends together, or immediately go back to playing with eachother once we get home. My children are still young, though, but hopefully this will continue for many years to come.
Why do you think you didn't play with your two brothers very much? What ages in your life are you referring to? I think this is a really interesting topic.