a) Transfer of ownership through GPA(general power of attorney) unless registered does not hold any value,given judgement of hon'ble Apex Court in October 2011. Hence, GPA can always be cancelled/challenged.
b) To take care of your interest it is suggested to get these documents (gift deed, GPA, Will) registered and stamped properly. Once the property is registered on your name your father will not be able to cancel the registered document and your sisters will have to deal through long legal battle to claim their share in this property.
c) These kind of facts cannot be hidden/will not stay secret for a long time. As a matter of right the way you are staying in your father's house, your sisters can also come and stay but if the gift deed is registered and property is transferred on your name , you deserve the right as to who stays in your house.
Best Of Luck - Stay blessed.