Under section 11 of the Hindu Marriage Act, 1955, the marriage may be declared null and void if:

Section 11 of the Hindu Marriage Act, 1955 makes a marriage void only if it contravenes section 5(i), (iv) or (v): bigamy, prohibited degrees of relationship, or sapinda relationship. So a marriage where "the parties are within the degree of prohibited relationship" (unless custom permits it) may be declared null and void.

Unsoundness of mind and recurrent attacks of insanity are conditions under section 5(ii); breach of them makes the marriage voidable under section 12(1)(b), not void under section 11.