I'm familiar with beach volleyball, but in a first moment I didn't recall Conde Senior in my mind, he's really good for his age (double-underager). So there's Conde's son, Aleksander Śliwka's brother, Paolo Porro's brother, Dante Boninfane's son, Vigor Bovolenta's son, Vlado Nikolov's son, and maybe Vlasov and Kirillov from Russia have something in common with Ilya and Irina, but I'm not familiar with that. There's also Bergmann's brother as OPP in Brazilian team. After all, volleyball is a family sport.
There's also Erik Stoev, son of Martin Stoev, who is the head coach of the U19 NT, the former head coach of the senior NT, and a former NT player.