The Williams sisters
We know, we know. Venus and Serena aren't brothers. But when it comes to sibling rivalry on the courts, you can't deny that these two sisters are fiercely competitive. They've played each other in Grand Slam events 12 times, and Serena holds a 13-10 edge in career showdowns at WTA Tour events.
Serena has 47 career victories, while Venus has 44.