Bike registered on elder brother's name but younger brother rides it

Yes.
if you have valid driving license,you can drive any vehicle categorised under Motor Vehicle Act. It does not matter who is the registered owner. If owner gives her consent to drive.
In real many times you have seen driver of cars,buses,trucks,school vans are not owner they are driving on contract basis. I.e. Uber,Ola cabs mostly driver of these cabs are on salary plus incentive basis .
So you can drive vehicle even if you are not registered owner , but you must have valid driving license and other documents I.e. insurance, pollution control certificate etc.
And you must have consent from registered owner to drive vehicle .