1. If the same Sy. No. having an extent of 10 acres of land which your great grandfather owned and the same Sy.No. your father has inherited, then it can be considered as Ancestral Property to you and your siblings. However, if your father has inherited your grandfather's self acquired property of 10 acres, then the property will have the characteristics of self acquired property in your father's hands.
2. Based on the above, if it were to be a self acquired property of your father, then it's his prerogative to deal with the property in any manner he likes, including gifting the property to anyone son or giving the property to all his children equally, including his daughter. In that case, your father will be well within his rights to give the property to only one son. The other siblings, your three brothers and one sister can question this in court but being this property treated as self acquired property of your father, your other brothers and a sister will not have any birthright in your father's self acquired property during your father's lifetime.
3. If using the yardstick explained under point (1) here and if it were to be treated as self acquired property, then your sister will not have any right.
4. Your father has to obtain NOC from Bank before gifting the property to anyone son, as there has been charge created on the property by Bank and the Bank will have first charge over the property and your father would have surrendered his rights over the property till the housing loan is discharged. Still if your father persists to execute a registered Gift Deed in favour of his one son, during the pendency of loan, the Donee will not get absolute title to the property.
5. If it were to be your father's self acquired property, he will be free to execute a registered Gift Deed to anyone son. When the charge is created on the property, the Mortgagor has to obtain NOC or the Mortgagor has to clear the loan.