Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- Every committed Muslim should accept Allah’s Commands willingly, even without realizing the wisdom behind the Divine rule in question.
2- If man manages to realize the wisdom behind certain prohibited things, it is well and good.
3- If his mind fails to comprehend that Divine Wisdom, then he should accept the truth as it is, keeping in mind that Allah Almighty is the All-Knowing of what is beneficial to His Creatures.
4- When Allah Almighty prohibits something for His servants, He does so certainly out of His overflowing mercy on them, to save them from all that may harm them. He is indeed Ever-Kind and Most Glorious!
5- Your concern for your sister-in-law is commendable, but you are prohibited from marrying her while you are still married to her sister. You could make more efforts to find a suitable husband for her, even someone who is already married or has been previously married.
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?
Allah Almighty knows best.