Among the Muslims marriage can be declared null and void when either of the contracting parties in insane or is below the age of 15 and consent of the parent; has not been obtained. It is also invalid when a woman marries another person when her first husband is alive or when husband has manned fifth wife.
Marriage among very near relatives, such as real sister, brothers, mother, mother-in-law, grandmother etc. is invalid. Strictly speaking marriage with idol worshippers is also not valid. If a marriage is being performed among the Sunnis and there are no witness, the marriage will be irregular though that may not be invalid. A marriage contract dissolved during the period of Iddat is invalid. Marriage with a pregnant woman and while on pilgrimage is also not considered valid.