Case has been filed by a unknown person to us

My mothers brother got signed some blank promisory note during my sister marriage and Inturn got our flat registered on his name for the money he gave .After few years of business partnership my father found out my mama to be fraudulent and had a fight with him.As a revenge my mama used my fathers signed blank note which we forgot about and wrote some other person name as money lender to my father and filled up the whole promisory note .the case has been filed a year and they gave our old address so that we would not get a notice of happenings.after an year suddenly we got a notice to the current address to pay to the unknown person.We don't even know any details of the person who filed except my mama who signed as witness.we don't even have a chance to pay n settle out of court as we don't know the person.my father is suffering from liver failure he doesn't know much abt law.we daughters have to fight.This case has been filed only to defame out of mere jealousy.Paying the amount is not a matter but how to pay to a unknown 1.how can we fight this fraud case 2.can we file criminal conspiracy case if so under which section