Your 6-year-old niece does not like to play with boys. Your brother thinks that this is okay but his wife thinks that your niece should play with both boys and girls

What can you tell your sister-in-law about gender differences in styles of interaction and how that is related to the selection of same-sex playmates?
What will be an ideal response?

A good answer will be similar to the following:
In many cultures, girls generally play with girls and boys play with boys. Even when parents encourage boys and girls to play with each other most children will resist. Part of this resistance comes from the fact that boys and girls have very different styles of interacting with peers. Girls' remarks and behaviors tend to provide support for each other — enabling interactions. Boys' behavior tends to be rougher and boys try to dominate each other by using intimidation, threats, and exaggeration — constricting interactions. When boys and girls interact, girls find that their enabling style is ineffective with boys. So, your niece is not unusual.