Where it might start getting odd is.. like in China, it's very rare to have a sibling. If people are choosing the sex of their children and most of them are opting for one girl and one boy, you'll get fewer girls who can say they have sisters and boys who say they have brothers. That bond of 'sisterhood' and 'brotherhood' may become rather abstract in a society where it's rare.

If it only happens in the case of twins and those opting for more than two children.

Hey, I just thought of a science fiction example of gender selection. Ender's Game. That was just one component of trying to breed a military genius.