Can we annul a marriage on grounds of mental instability?

as per valid hindu marriage registration is not mandatory, but performance of rituals is essential. consummation of marriage is also an ingredients of a valid marriage. if you can prove her mental disorder medically only then it is advisable to take this as a ground, otherwise non consummation and cruelty may also be grounds for divorce.

Hi If the girl is proven medically to be mentally unstable then your brother can go for divorce and courts will be inclined to grant divorce on the grounds of mental instability. It is not necessary that the marriage should be registered for filing of divorce. According to Hindu marriage act, the performance of rituals such as talking seven rounds the fire, tying of mangalsutra, garlanding the woman is sufficient proof of a marriage. If you have strong evidence that the girl is mentally unstable, go ahead and file for divorce.

There are enough grounds in your case to get divorce. There is no need of consent of the woman. Do not hesitate to contact me in case you need any clarification/assistance.