Legal heir certificate

Last year my elder sister passed away. before death she made will all property and employment benefits to her son. her husband applied and got legal heir certificate. last 16 years my sister son staying with my mother. now my sister son going to join college. so, he trying to get money from his my mother saving account and jewelry in locker. he requested his father regarding legal heir certificate but my sister houband not ready to give. Tried to applied xerox copy in R I office. But my sisterhusband ordered RI don't give legal heir anyone. Then how my sister son get legal heir certificate. ,? Or Shall he Apply new legal heir certificate. Help me