Is it permissible to marry two sisters who are not full sisters at the same time, and if not, how did the Messenger of Allah, peace and blessings be upon him, marry Maymunah bint Al-Harith and her sister (…) bint Khuzaim?

It is not permissible to combine two sisters, whether by blood relation or by suckling, under the marital bond of one man, because Allah Almighty says: "And that you take [in marriage] two sisters simultaneously." This is a matter of consensus. However, it is permissible to marry the other sister after divorcing the first and her waiting period has ended, or after her death, as the Prophet, peace and blessings be upon him, did when he married Zaynab bint Khuzaymah al-Hilaliyyah, and then after her death, he married her maternal sister, Maymunah bint al-Harith al-Hilaliyyah.