(Part No. 18; Page No. 228)
It is impermissible to
marry two sisters at the same time, for Allah (Exalted be He) stated in clarifying the women unlawful for marriage:
...and two sisters in wedlock at the same time
Divorcing the younger sister to marry the oldest one, or continuing with the former and leaving the older sister is something that should be weighed. If you see that it is in the best interest of everyone concerned for you to divorce the younger sister and marry the older one in order to raise your nephew, or for another reason, it is up to you. However, if you feel that staying with the younger one is better for you, then keep her. In any case, regarding this matter, you should refer to your kin and consult them lest you should divorce the younger sister, then they refuse to give you the elder one in marriage. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.