Since your wife's father died intestate, that is, without making a will, all the legal heirs, including your wife, her brother and her mother have equal right over her father's assets. But if she is forced to go to the Court then :
1. Your sister would have to obtain a Succession Certificate and file a suit of partition in the Court.
2. The time limit to file a partition suit is 12 years as per Article 65 of the Limitation Act.
3. At the time of filing the partition suit, your sister can seek mandatory injunction on not just the assets known to her, but also request for restraining her bother and mother from withdrawing any amount from any unknown bank accounts and/or share one third of the proceeds from any other assets presently unknown to her. Once the respondents i.e. your wife's brother and mother join the proceedings, the Court would ask them to share details of all assets known to them.