As far as Islamic Shari`ah is concerned, it is not allowed for a Muslim man to take two real sisters as co-wives even if they agree.

Sheikh Yusuf Al-Qaradawi, in his well known book, the Lawful and the Prohibited in Islam, stated that: “As opposed to the practice of the period of jahiliyyah, 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 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. (Reported by al-Bukhari and Muslim.) and he said, If you do this, you will sever your ties of kinship, (Reported by Ibn Hibban.) and how could Islam permit the breaking of such kinship ties when it places so much importance on them?