Harrasment by my aunt

Dear Sirs,

1. My father purchased a land of 266sqyds, and got it registered in the year 1985. In support of finances, at the time of purchase, my father has borrowed an amount from my aunt(my fathers elder sister).
2. As he was not in a position to repay the borrowed amount over a period of time, he has promised his sister and brother-in-law to register half of the land on their name and no written agreement was made in this regard.
3. In the year 1995, my dad has requested his sister to get their share of land registered on their name, but they didn’t accept the proposal to register at that time, stating that my uncle is a govt employee and tax issue may arise. So my dad availed loan and constructed the house in his share of land.
4. In the year 2009, after my dad retirement, my dad made a gift deed in the name of my aunt for a land measuring only 120sqyds(against 133sqyds promised earlier) after discussions and mutual agreement.
5. My aunt, further tranfered the same land measuring 120sqyds to her married daughter in the same year i.e 2009. at present as per the EC the land is in the name of her daughter.
6. After a lapse of 1year, in the year 2010, my aunt along with several elders of our family approached my father and started demanding money for the balance 13sqyds land, and a settlement amount was paid by my dad through cheque to my aunt upon elders advise,  and written statement was made mention that cheque was recevied by her and also stating that the amount paid is in support of construction of house in their share of land .
7. From then, there was complete breakup between our families, and also they didnt even constructed any thing in their share of land.
8. Now, at present after a lapse of nearly 7 years from date of transferring the property(which is not in her name at present,) my aunt again started demanding for money and black mailing/insulting/humiliating us by writing several letters to our colony president/relatives/my fathers work place/ my work place stating that my dad has cheated her, and also threatening for legal action against us.

So, someone please kindly, guide me on what steps I need to take to stop her from harassing my father.

What legal actions can she really take, is there any mistake my father has really done. Is there any risk to our property which is in the name of my father, if yes what steps need to be taken to save our property.

Please guide.