Hi
1) If your paternal aunt has made a registered will in your favour, then your father or your sister/brother in law does not have any title over the property.
2) Since there is no title, your father cannot execute any gift deed on the property. Even if any Gift deed has been made, the same is null and void in eyes of law.
3) So you have to file a suit for cancellation of Gift deed and
4) Also get the will executed by your aunt in your favour probated in the court of law.
Once items mentioned in 3 and 4 are done, you can be free from all the unwanted nuisance so created by your sister.
5) For probate of will, you need to pay to the court in the form of court fees a stamp duty of about 5% of Fair market value of property.
6) Time lines for cancellation of Gift deed and Probate of Will will be approximately 12-18 months depending on the Pecuniary jurisdiction of the courts in which you need to file both of the above suits.
Hope this information is useful