Are my wife’s sisters mahram?

Your wife’s sisters are not your Mahrams. Social interaction with them is not permissible.

Rasoolullah Sallallahu Alayhi Wa Sallam forbade non-mahrams (unrelated men) to enter upon women. He Sallallahu Alayhi Wa Sallam said: “Beware of entering upon women.” One of the Sahaabah said to him, “O Messenger of Allaah, what about the brother-in-law?” He said: “The brother-in-law is death!” (Reported by al-Bukhari, Fath al-Baari, 9/330).

The expression “the brother-in-law is death” may have a number of meanings, such as the following:

 That being alone with a brother-in-law may lead to religious doom if it results in sin.

 That it may spell disaster for the wife if her husband’s attraction to her sister leads to divorce.

 That being alone with a non-mahram woman is as terrible as death.

And Allah Knows Best