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.
If the person who intends to marry your sister is pious, trustworthy and has a good character, then it is recommended not to refuse such a person. The Prophet said: “When someone with whose religion and character you are satisfied asks you to marry him, accept his request and marry him. If you do not do so, there will be corruption and great evil on the earth.” [At-Tirmithi and Ibn Maajah]
The color of a man or his nationality have no weight in the Sharee’ah. Allah Says (what means): {Indeed, the most noble of you in the sight of Allah is the most righteous of you. Indeed, Allah is Knowing and Acquainted.} [Quran 49:13]
Therefore, we advise you, if you are convinced that he would be a good husband for her, to try to convince her parents gently and wisely. If you fail to do so, tell your sister to do the following:
First, she should try to explain to her father that she is inclined to this man and that she is convinced that he would be a good husband for her. In addition, there is no legal reason for him to refuse this man's proposal since the honorable and preferable person is the one who is pious and follows the Sharee’ah.
Second, if her father does not change his mind and persists on his refusal, then she might tell him that he is going against the rulings of the Sharee’ah by refusing that man without any legal reason. In fact, such a refusal would be considered preventing a daughter from marriage, which is forbidden in the Sharee’ah.
Third, if this does not convince him, then she may plead in an Islamic court, if it is available, otherwise she may bring her case to any available substitute to an Islamic court such as Muslim authorities. Indeed, the case will be in favor of the girl.
All the above suggestions are allowed to her only if the person is pious and has a good character. If the person is not so, then it is better for her not to involve herself in a conflict with her father, and surely Allah will give her a better husband instead of this one, Allah willing.
On the other hand, we advise her to take into consideration the following matters.
First, one should not undertake any action before performing the Istikhaarah prayer as the Prophet instructed us.
For more benefit on the Istikhaarah prayer, please refer to Fatwa 81434.
Second, a girl should know that the consent of the guardian is compulsory for a valid marriage contract. The Prophet said: “No marriage can take place without the presence (and consent) of a guardian and two reliable male witnesses, and any marriage that was not done in this way is nil and invalid, and if they have dispute, then the Sultan (ruler) is the guardian of anyone who does not have a guardian.” [Ibn Hibbaan]
There is no contradiction between this Hadeeth and what we mentioned before because the guardian is restricted to act according to the Sharee’ah and whenever he misuses his authority and prevents the girl who is in his guardianship from marrying, then a Judge of an Islamic court will force him to marry her or he, the judge himself, can marry her to the suitable person.
Third, the woman should beware of having any kind of relation or contract with any non-Mahram man. The consequences of illicit relations are more harmful than anybody could imagine. Satan gradually induces the person in committing what is forbidden until he destroys him.
Allah knows best.