This property was purchased by your father, though he did not got it registered. However upon his intestate death the property shall devolve upon all his legal heirs equally.
For this you can obtain legal heirship certificate from the revenue department which will suffice the requirement.
In case there is not provision of granting legal heirship certificate in Delhi, then you may try for succession certificate.The NOC no doubt will be sufficient enough for registering the property now, but when here is a alienation in the future, the buyer shall be insisting on it.