Request advice

Dear Sir, My sister has got married since from Dec 2010. It was a fully arrange marriage. Her in laws have made a big fraud with us by keeping away the facts of their son. Again after marrige She has been tortured in both physically & mentally by her in laws & Sister in law for dowry.She remain silent for 5 years with a hope that things will be ok in future. Her husband was a pyscho patient. again Her husband is infertile hence she can not have baby in life. Now we came to know that the truth was known to all inclusive of her in laws , Husband & sister in laws 20 years before.They made an excellent script and succesful in doing our marriage. The fact came to her notice few days back by her husband. The doctor who has did the tests 20 years back provided us the confirmation. Now My sister's in laws were not accepting the truth & they have throw away my sister & her husbnad from their house by doing a false alegation in local police complainting physical torture to them. Now her in laws planning to give all their propert to their Daughter Son In laws. My sisters life has been spolied & now sister wants to file case against her in laws & sister in law. suggest your legal advice.