Co operative society

My husband Ketan passed away leaving me his wife as nominee. I want to transfer this flat on my and my mother in law name as we are the legal heir. But the society is not agreeing to it. Saying it will get transferred to nominee name only. Kindly let me know what can I do to make my mother in law enter the property as legal heir. And if I take legal heir certificate what will be the court fee As I have to take from high court.