Can ownership become 50-50 if one party contribute only 33% funds

Hi, My mother bought a flat in Mumbai 5 years back.Since she was short of funds my brother in law contributed 33% on it by taking a loan against the property.Both my mother & brother in law have their names on the property title while i been one of the sibling have my name as a nominee.Since we have no written agreement on share holdings,my brother in law is now asking for a 50-50 share.Also we are 5 siblings in all 1>In the event of death of my mother who has the legal rights on the property?in case we have no Will for my mother 2>If my mother has a registered Will assigning me as a whole successor can i then claim my mother share on the property 3>How can we prove that the actual share holding by my brother in law