I think it’s totally normal to have a preference for one or the other sibling, especially as your family size increases. And the great part about it is, you can switch allegiance at will (ie hang out with one sibling one time, or side with one sibling per issue). I have both male and female siblings and gender doesn’t play into favoritism, but I guess I must acknowledge that there are two sisters whom I am the closest to, and I believe it has to do with similar ideals and ethics, not necessarily similar interests.