I suspect that they might be brothers but not real ones. Probably half-brothers. Maybe Kabir is Anupriya and Vansh's father's child from an affair, but he refused to accept him as he already had a family. That is probably why Anupriya killed Uma Raisinghania to take her place and get legitimacy for Kabir. Also, probably why they think that everything that vansh has is kabir's in some twisted way. Just speculating 🤷♀️