Parents property taken over by son. How to get them their house back?

The option available to you is to file a suit for partition and permanent and mandatory injunction. You cannot claim the entire house back, but the share can be claimed. If your wife’s brother wishes to retain the house then he shall have to pay them the value of their share or else the house shall be sold and the proceeds shall be divided as per the share.

Moreover your mother-in-law has legal right to maintenance from her son under: 

1. CODE OF CRIMINAL PROCEDURE: Section 125(1)(2) makes it incumbent for a person having sufficient means to maintain his father or mother who is unable to maintain himself or herself and on getting proof of neglect or refusal, may be ordered by a first class magistrate to make a monthly allowance not exceeding Rs.500/-. It is applicable to all, irrespective of their religious faith and religious persuasions, and includes adoptive parents. This section has been interpreted by the supreme court in its ruling so as to make daughters and sons, married or unmarried, equally responsible to maintain their parents.

2. THE MAINTENANCE AND WELFARE OF PARENTS AND SENIOR CITIZENS ACT, 2007: A senior citizen including parent who is unable able to maintain himself from his own earning or out of the property owned by him, is entitled to get relief under this Act. Children/grand children are under obligation to maintain his or her parent either father, mother or both.[13] Likewise, relative of a senior citizen is also bound to look after the senior citizen. If such children or relative is not maintaining his parents or senior citizen respectively, then the parents/senior citizen can seek the assistance of Tribunal constituted under this Act, to enforce the remedy of maintenance. Such parents/ senior citizen can file an application before the Tribunal, claiming maintenance and other reliefs from their children/relatives as the case may be.

3. HINDU ADOPTION AND MAINTENANCE ACT: By Section 20(1)of the act, every Hindu son or daughter is under obligation to maintain aged and infirm parent, if unable to maintain himself or herself is entitled to maintenance. Amount is determined by the court taking into consideration the position and status of the parties.