Joint property share

My husband and his parents has a 2bhk flat in joint loan.his parents paid 37 lac as down payment at the time of purchase(black money).however my husband never stayed a single day in the flat once we got married as he got a transfer to other state.since 5 years he is paying 25 thousand as the emi and since 3 years he is also paying house rent in other state of another 25 thousand.Till now he paid 15 lac emi for the mumbai flat and 15 lac as rent in other state.his parents rent off their old flat and paid the remaining part of the EMI from their old flat.Now my husband can't afford to pay for both so he asked his parents to sell off the flat and distribute the share.but his parents are sticking to their 37 lac down payment and according to their legal advisor our share on the property will be watever emi we paid till date plus some interest which comes around 18 lac which we obviously disagree.current bank loan is 36 lac pending and since 3 months we stopped paying the emi as we want our right share.the total emi for the flat is 45 thousand.his parents don't want to share the flat and neither they consider the fair market value.according to the housing sites the current market price of the flat is 1.5 crore.can u please tell us what's legally the share of my husband?.we consulted a lawyer here and according to him the current flat should be on rent as we never stayed a single day.i just want to know what will be legally my husband share in the property?my husband has a younger brother as what will be his ownership in the property post his parents if no will is made?