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 on being married to a woman and her sister
at the same time is mentioned in the Qur'aan. Allaah
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"
[al-Nisa' 4:23]
And it is stated in the saheeh 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-Bukhaari,
4821; 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.
Shaykh Ibn 'Uthaymeen (may Allaah have mercy on him)
was asked about a woman who lives with her married
sister and does not wear 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. Allaah 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.
23. 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, Allaah is Oft-Forgiving, Most
Merciful"
[al-Nisa' 4:22-23]
Allaah 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 Allaah says: "and two sisters in wedlock at
the same time". The case of the wife's sister is not
as the questioner understands it.