I remember this clearly, but not sure about the episode. Nani and mami was having Conversation and nani say Dhruv will come. Probably he is Akash's brother.
I've always wondered about this too. He wouldn't have been Arnav and Anjali's brother as they showed the flashbacks with just the two from the beginning.
But later Mami went on to say Akash was her only son during the Payash wedding track.