All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Combining two sisters in marriage is strictly forbidden. Allah Says while enumerating the women who are forbidden in marriage (what means): {…and two sisters in wedlock at the same time, except for what has already passed…} [Quran 4:23]
The Prophet also forbids it. Al-Bukhari reported that Umm Habeebah (one of the Prophet's wives) said to the Messenger of Allah: “O Prophet of Allah! Marry my sister the daughter of Abu Sufyaan.” He said: “Do you like that? She said: “Yes; the person that I like most to share the good with me is my sister.” Thereupon, the Prophet said: “This is unlawful for me.” Then, he said: “Do not offer me your daughters or sisters.”
What is meant by not combining sisters is avoiding to have two sisters as wives at the same time even if each one of them lives in a remote area from the other. This is also one of the issues on which all the Ummah has agreed.
Now, if the husband divorces his wife and the ‘Iddah of the latter expires, then he can marry her sister.
Allah knows best.