False accusations against us.

The husband and in-laws of my sister in law (nanad) has filed a divorce and has falsely accused my husband and me that we forced her to do this marriage. My sister in law has a low IQ level and she was living with us. She also was on medication and when my mother in law died she became more depressed. But whenever we asked her to do household chores she did it, she has always been a bit lazy but she never denied anything we asked her. She wanted to get married but we were unsure if she will adjust or not. Then one of our known people send us this guy's biodata in which they mentioned that he has his own house and monthly earns well. They came to our house and we cleared my nanad's condition. They also talked to her personally and that day only the finalized the marriage saying their tau ji was ill and he wanted to see his marriage while he is alive. We also went to see his house. Then we had roka ceremony too when all their relatives came and after 15 days they got married. After that they said they will stay in same colony , his Bhai and bhabhi (in-laws) said they will sell the old house (which was not even his, we came to know later) and quit the old job (which we have no evidence of)and start new by living with them. When everything got final her in-laws started calling us and often complaing about her. After 2 months her husband left her at our home saying she is mentally ill and he want divorce. After that we came to a settlement and we were ready to divorce but law said only after 2 years not before that so we waited . She was again living with us . For 10 months both the families had no contacts with eachother. After 10 month her husband came one day saying his bhai bhabhi sold that old house which was his tau's house who had no children. Tau has given the power of attorney to my nanad husband who is illiterate so he handed it over to his bhai bhabhi who upsurge the whole property and forced him to buy their second brother's house if he wants to stay with them. So now he wants to live with my nanad again. After staying 2-3 times at our home he took her with him. Again after one month he left her at our doorstep saying she is mentally ill and he wants divorce. After few days we went to their house to talk but they misbehaved with us and threatened that they filed a divorce and their lawyer will talk now. After a month we got a legal notice of divorce and a chargesheet full of false and serious accusations against us. They have blamed us for conducting a forceful marriage and that my nanad has accused us too for many things and if they proved all these things right then court should set enquiry against us. But the thing is my nanad has also accused them for so many things at our home that her husband made her drink alchohol and also attempted marital rape & other things. So what can we do as they tried to defame us and told us wrong about his job and property which now they showing him unable. Can we file case for fraudulent and defamation?