Is it allowed in islam to marry a woman who is the direct sister of the man whom you sister is married to?
God is very clear on who is and is not permissible for a Muslim man to marry. As explained in Surat an-Nisa:
[4:22] And do not marry those [women] whom your fathers married, except what has already occurred. Indeed, it was an immorality and hateful [to Allah ] and was evil as a way.
[4:23] Prohibited 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 [milk] mothers who nursed you, your sisters through nursing, your wives' mothers, and your step-daughters under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you. And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allah is ever Forgiving and Merciful.
[4:24] And [also prohibited to you are all] married women except those your right hands possess. [This is] the decree of Allah upon you. And lawful to you are [all others] beyond these, [provided] that you seek them [in marriage] with [gifts from] your property, desiring chastity, not unlawful sexual intercourse. So for whatever you enjoy [of marriage] from them, give them their due compensation as an obligation. And there is no blame upon you for what you mutually agree to beyond the obligation. Indeed, Allah is ever Knowing and Wise.
Nowhere in this list is a brother-in-law's sister mentioned, or any directly analogous variation on relationship-through-marriage: the general ruling given by "…And lawful to you are [all others] beyond these…" would indicate that marrying her is permissible.
Generally yes, keeping in mind the following special cases in which you can't marry her:
She is your sister in Milk, i.e, her mother breastfed you or your mother breastfed her, this is because the prophet said:
The Prophet said about Hamza's daughter, "I am not legally permitted to marry her, as foster relations are treated like blood relations (in marital affairs). She is the daughter of my foster brother."
حَدَّثَنَا مُسْلِمُ بْنُ إِبْرَاهِيمَ، حَدَّثَنَا هَمَّامٌ، حَدَّثَنَا قَتَادَةُ، عَنْ جَابِرِ بْنِ زَيْدٍ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم فِي بِنْتِ حَمْزَةَ " لاَ تَحِلُّ لِي، يَحْرُمُ مِنَ الرَّضَاعِ مَا يَحْرُمُ مِنَ النَّسَبِ، هِيَ بِنْتُ أَخِي مِنَ الرَّضَاعَةِ ".
She is the sister (or half sister) of a woman that you'r married to.
She is your half sister, this can happen, for example, if your father (let's call him Saleem) and mother (called Mary) divorced after having you and your sister Fatima, Saleem married and had a new boy called Salem and a new daughter called Salma, Mary married and had a new girl called Miriam and a new boy called Mario. In this case, Asma and Salem and Miriam can marry, and Mario and Salma can marry, but YOU still can't marry Salma and Miriam as they're your half-sisters; and Fatima still can't marry Mario and Salem, as they're her half-brothers.
If none of the above special cases is your case then yes, you can do marry her, as she's not of the women that you can't marry.
Please note that there might be other special cases I didn't think about, you should just seek all Milk, blood, and in-law routes to her and see if any existing route prohibits your marriage, if none, then you can.