Relinquishment deed v/s sales deed

Law Gurus,

There is a flat in a society in Mumbai suburbs. It was on joint name of my Father and Mother.

My father passed away recently leaving behind my mother, myself (son) and my married sister.

Nomination papers have my name as the nominee.

From my Fathers 50% share inherited to 3 of us, my mother and sister are willing to release their share to me.

What is the cheapest, shortest yet legally correct procedure to do this?

As said, the property is in a society bound by Maharashtra Co-Op Societies Act

Thanks for your advice. 

Regards,

MS