Though they were both brought in up France, they are of Senegalese descent and Sonko insists they are related - even if he doesn't know how.
"I'm not sure if it's through my mother or father's side and I only found out recently. When I told my brother he said he already knew. But I've never met or spoken to him and, believe me, it was a surprise when I found out."
One of the main reasons their paths have never crossed is because Sonko comes from an enormous family.
"I have no idea how many cousins I have," said the defender who was born in the Senegal city of Bignola. "I knew I had a cousin somewhere called Sagna. So I asked my mother if it was the one who plays for Arsenal and she said it was.
"In fact, I've also got another cousin called Sagna who was on trial at Barnet recently. Now he's definitely my cousin, but my brother has been telling me the Arsenal one is my cousin too. He said to trust him and that we are definitely related.
"I've spoken to my parents about it and I've told them I will say hello. Apparently he knows about me as well and I guess we'll find out more on Monday."