1. After the demise of your father, intestate, his mother being your grandmother also becomes one of the legal heirs of your father along with his wife and children whether you like it or not,
2. So, for transferring the 50% share of your father in the said joint property in favour of your younger brother, NOC of all other legal heirs including that of your grandmother are required to be submitted before the HUDA authorities,
3. If you can not collect he said NOC from your grandmother wait till her demise intestate.