All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
When the wise Sharee'ah deemed polygyny allowable, it enjoined the Muslim man to treat his wives on a footing of equality. Allaah, The Exalted, says (what means): {And if you fear that you will not deal justly with the orphan girls, then marry those that please you of [other] women, two or three or four. But if you fear that you will not be just, then [marry only] one or those your right hand possesses. That is more suitable that you may not incline [to injustice].}[Quran 4:3]
In fact, this is the only condition set by the Sharee'ah for the Muslim man who chooses to practice polygyny. We know of no evidence suggesting that Islamic Sharee'ah obliges the husband to inform his first wife of his intention to have a second wife. Please refer to Fatwa 86395.
Refraining from informing the wife of the second marriage does not constitute deception on the part of the husband. He is entitled to marry another wife as per the Sharee'ah. In most cases, the greater interest entails that the husband does not inform the first wife of his intention to remarry so as not to hurt her and to spare her the unfavorable feelings of distress. This matter does not involve any tampering with the first wife's feelings.
In fact, polygyny abounds in many benefits that should not be missed merely because of a given wife’s refusal or aversion towards the idea. A woman is naturally averse to polygyny; however, it is not wise to let the wife's innate aversion towards having co-wives deny the husband the benefits of polygyny. For more benefit, please refer to Fataawa 86818 and 90132.
However, a Muslim wife should see this experience as any affliction that may befall her. If she is patient, this will be a cause for the raising of her rank and atonement of her sins. Please refer to Fatwa 83577 about the merits of patience.
If the husband informs his wife of his second marriage or she finds out about of it, it is not permissible for her to seek divorce merely because her husband has another wife. But if his second marriage incurs considerable harm on the first wife, she is entitled to ask for divorce or Khul‘ because evident harm is one a legitimate reasons for seeking divorce. Please refer to Fatwa 131953.