Mental illness is a heavy cross to bear, no question. Whether you are someone who suffers from it, or a loved one trying to help someone cope with some form of mental illness, it can affect nearly every aspect of normal life and you may find yourself in situations you never thought would be possible, like divorce. However, determining unsoundness of mind can be a difficult issue, especially when made in retrospect.
A Hindu marriage is voidable according to this law if either party:
Is incapable of giving a valid consent as a consequence of unsoundness of mind or
Though capable of giving a valid consent has been suffering from mental disorder of such a kind or to such an extent as to be unfit for marriage and the procreation of children, or
Has been subject to recurrent attacks of insanity. In the above-mentioned three circumstances, the marriage can be held null and void. Additionally, if the marriage has not been consummated due to impotence of the respondent, the marriage can be considered null and void. Also, the marriage can be considered null and void if that the consent of the guardian was obtained by force or by fraud; or the respondent was at the time of the marriage pregnant by some person other than the petitioner.
Therefore, your brother can decide for filing a petition for annulling his marriage on the basis of her past medical records.