They know how to listen
"Men with sisters tend to be better listeners," says Murray. "By and large, women use more words than men do. If you're around people who talk more, then you're exposed to a lot more words. But boys, as a consequence, may not express themselves so much through words, but through behavior and their physicality.
"Words are less relied upon to communicate with other boys. When you have a mixed-gender household, you're fusing these two strategies together. As a result, boys with sisters are exposed to more words and are more likely to be good listeners."
They understand we are human
As women we are often made to feel like we have to be superhuman; meaning we don't show traits of being a living, breathing thing -- lest we become entirely unappealing and borderline gross.
I'll give you two examples: One boyfriend, who had an older brother, made it clear early on that he was wildly turned off by women who alluded to the fact that they had to use the bathroom. Apparently women have this magic ability to wish away any aftereffects of a night of cheap beer and chicken wings with their boyfriends, so that we are powder fresh 100% of the time. I was not aware I possessed this power, but I learned real fast. It was a long, long four years.