This property originally belonged to your mother, registered on her name.
Therefore upon her death, if she had not transferred the property to your father's name, the property becomes eligible for intestate succession only. Your father cannot assume to have got the entire property to his name and make a Will to transfer the entire property to the person of his choice.
Your father can transfer only his share in the property to the person of his choice.
If you are the named beneficiary of the Will and even if the Will had become enforceable, until and unless the other legal heirs have executed a registered release deed relinquishing their rights in the property,. you may not be able to claim clear and marketable title to the property, hence this hurdle cannot be cleared for sale of property until the observed irregularity is rectified.