These two persons have to repent to Allah from such a heinous crime. Zina incurs the wrath of Allah and its evil consequences pursue its perpetrators in this life and in the Hereafter. If this husband is not satisfied with his wife, he may seek a second wife other than the sister of his wife.
As opposed to the practice of the period of jahiliyyah (pre-Islamic era), Islam forbade taking two sisters as co-wives at the same time because the feeling of love and sisterliness which Islam wants to maintain between sisters would be destroyed if one sister became the co-wife of the same husband. While the Qur’an mentioned the two sisters, the Prophet (peace and blessings be on him) added, ‘A man may not be married to a woman and her paternal aunt (at the same time), nor to a woman and her maternal aunt.’ (al-Bukhari and Muslim) And he said, ‘If you do this, you will sever your ties of kinship.’ (Ibn Hibban) Then, how could Islam permit the breaking of such kinship ties when it places so much importance on them?