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What makes a fighter? Is it nature? Nurture? Or a combination of both? No one knows for sure, and sibling fighters only confuse the issue further.

In MMA's 19-year history, there have been 20 sets of siblings that have competed at the highest levels—in the UFC, Pride, Strikeforce or all three.

Sometimes both brothers have had significant success. Sometimes, one has stood out, and the other comes along for the ride.

Some familiar names grace this list. From the Hughes to the Gracies, from the Nogueiras to the Serras, MMA's most famous families are all included.

Which brothers stand out from all the others as the top family affair in MMA?

Read on to find out.