When an elder brother is alive, the younger brother gets the title of yuvan , being the descendant of a grandson etc.,
Even when a वंश्य like father &c is not alive (and a brother is not vansya), the younger brother gets the designation of yuvan, when the elder brother is alive; thus:
गर्गः (dead) -> गार्गिः (dead) -> गार्ग्यः (dead) -> गार्ग्यः, गार्ग्यायणः
The word वंश्यः is confined to ancestors like father &c, so a brother can never be a vansya, because he is not the source from which the other brother arises. This sutra applies to cases when Vansya is not meant. Thus Gargyayna is the name of the younger brother with reference to the elder brother, Gargya; similarly Vatsyayana, Dakhsayana, and Plakshayana.,