If your wife and her sister want to hold the property from being alienated or fraudulently divided by your wife's sister in law enforcing her evil design, your wife has to file a suit for partition and an application in it seeking to restrain her brother/sister in law from alienating or encumbering the property in any manner till the disposal of the main suit.
Your wife can give a power of attorney deed to any close relative in India to file the suit on her behalf so that the suit can be filed in her absence also and that too immediately.
She should not show that knows about her brother's mental ill-health, because one cannot sue a mentally challenged person. Your wife's sister can join your wife to file the suit jointly.
Both your wife and her sister are entitled to one third share respectively in the properties left behind by their parents upon their intestate death.