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.{C}
There is no harm for a man to marry his wife’s maternal cousin, because she is not mentioned in the Quran or the Sunnah among the forbidden women to marry. So, she is included in the Saying of Allah (which means): {And lawful to you are [all others] beyond these.} [Quran 4:24]
What is forbidden in marriage is to combine between marrying a woman and her paternal or maternal aunt, or a woman and niece. Other than this, then there is nothing that prevents a man from marrying any other relative of his wife’s.
When explaining the Hadeeth that reads: “A man cannot be married to a woman and her paternal aunt, or a woman and her maternal aunt at the same time.” An-Nawawi said in Sharh Muslim: “As regards other relatives, such as being married at the same time to a woman and her paternal cousin, or a woman and her maternal cousin, and the like, then this is permissible according to the view of our School and the view of all the scholars.” [End of quote]
Therefore, it is permissible for you to marry this woman if her husband divorces her and her waiting period ends.
However, be careful not to be the reason of her divorce from him; because there is a stern warning about this, as in the Hadeeth narrated by Abu Hurayrah that the Prophet said: “Anyone who incites a woman to be divorced from her husband (so that he would marry her) is not one of us.” [Abu Daawood]
Due to the seriousness of inciting a woman to get divorced from her husband, some scholars hold the view that it is forbidden for a man to marry the woman whom he incited to be divorced from her husband. Nonetheless, the majority of the scholars hold the view that it is permissible for him to marry her.
Regardless of the ruling, it is better if this woman can reconcile with her husband, as Allah Says (what means):
• {And if you fear dissension between the two, send an arbitrator from his people and an arbitrator from her people. If they both desire reconciliation, Allah will cause it between them. Indeed, Allah is ever Knowing and Acquainted [with all things].} [Quran 4:35]
• {… and settlement is best.} [Quran 4:128]
Finally, it should be noted that marrying a second wife is permissible in Islam, but one must be aware of its condition, which is the ability to be just between them; Allah Says (what means): {But if you fear that you will not be just, then [marry only] one.} [Quran 4:3]
Allah knows best.