Kevin Mac Allister signed for Union Saint Gilloise earlier this year.
Ahead of their Europa League clash, Klopp said: "Yeah about his brother and their father is here, obviously family is here. A big moment for him, definitely, I am pretty sure. They have another brother I think who is playing as well professional football. Cousins playing professional football. The father was a professional player, uncle was a professional football player.
"I think it is not the first time that family members [are] on the football pitch, I would be really surprised, but yes, it is special. I saw a little thing when they spoke to each other. Brothers love each other but have still fights from time to time, or at least how I learned it when I was young. I didn't have a brother, but two sisters and that was bad enough!
"I think it is a special, nice side story but probably, hopefully, not the main story of the game afterwards."