Is The Wife’s Sister a Mahram?
Question

Is the wife’s sister considered to be a Mahram?
Praise be to Allah.Is Your Wife’s Sister a Mahram?
The wife’s sister is considered to be a “stranger” (non-Mahram) to her sister’s husband, so it is not permissible for him to look at her, or be alone with her, or to shake hands with her.
Some people think that because she is forbidden in marriage to the husband that it is permissible to look at her and be alone with her and shake hands with her, but this is wrong.
What is meant by forbidden in marriage here is that it is not permissible for a man to be married to a woman and her sister at the same time ; similarly it is forbidden to be married to a woman and her paternal aunt or a woman and her maternal aunt at the same time .
The Prohibition of Marrying Two Sisters Simultaneously
The prohibition on being married to a woman and her sister at the same time is mentioned in the Quran. Allah has stated that among the women who are forbidden in marriage, as He says (interpretation of the meaning):
{and two sisters in wedlock at the same time.} [An-Nisa’ 4:23]
And it is stated in the authentic Sunnah that it is forbidden to be married to a woman and her paternal aunt at the same time, or to be married to a woman and her maternal aunt at the same time.  (Narrated by Al-Bukhari, 4821 and Muslim, 1408)
So what is forbidden is to be married to two sisters at the same time, and the wife’s sister is not forbidden to the husband for marriage in a permanent sense.
Misconceptions about Sister-in-Law as Mahram
Shaykh Ibn `Uthaymin (may Allah have mercy on him) was asked about a woman who lives with her married sister and does not wear the Hijab in front of her sister’s husband. She says that she is temporarily a Mahram (forbidden in marriage) to him. What is your response to that?
He replied:
This woman is confused. It is not permissible for her sister’s husband to marry her so long as her sister is with him, so she is forbidden in marriage to him for a certain period, not permanently. But her understanding is mistaken because those who are forbidden in marriage for a certain period are not Mahrams.
The Mahrams are those to whom marriage is permanently forbidden either because of blood ties or for a permissible reason, namely ties of marriage or ties created through breastfeeding. Allah Says (interpretation of the meaning):
{And marry not women whom your fathers married, except what has already passed; indeed it was shameful and most hateful, and an evil way. Forbidden to you (for marriage) are: your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters, your brother’s daughters, your sister’s daughters, your foster mothers who gave you suck, your foster milk suckling sisters, your wives’ mothers, your stepdaughters under your guardianship, born of your wives to whom you have gone in — but there is no sin on you if you have not gone in them (to marry their daughters), — the wives of your sons who (spring) from your own loins, and two sisters in wedlock at the same time, except for what has already passed; verily, Allah is Oft-Forgiving, Most Merciful.} [An-Nisa’ 4:22-23]
Allah did not say, “And the sisters of your wives.” What is forbidden is to be married to two sisters at the same time.
Based on this, we say to the sister of the questioner, who says that her sister speaks to her sister’s husband and does not wear hijab in front of him, and says that they are temporary Mahrams, that this is a mistaken notion and is not correct. This is not the matter of being temporary Mahrams , because what is forbidden is to be married to two sisters at the same time, as Allah says: “and two sisters in wedlock at the same time”. The case of the wife’s sister is not as the questioner understands it. (End quote from Fatawa Ibn `Uthaymin, 2/877)
For further insights, explore these answers:
Is the Father In-law a Mahram?
Is the father of one’s ex-husband a Mahram?
Is My Husband’s Stepfather My Mahram?
Is Brother-in-Law Mahram?
And Allah knows best.