The blessing of guidance is one of the greatest blessings that Allah, may He be exalted, can bestow upon His slave. It is obligatory for the slave who has been guided to show a great deal of gratitude to his Lord for this immense blessing. We are happy to hear of the repentance of those who have repented and of the guidance of those who have been guided. What has happened of that man being guided from kufr to Islam, and that woman being guided and brought back to Islam, and giving up living the life of the doomed people, is something that brings joy to our hearts and we ask Allah to make them steadfast in Islam and to set their affairs straight.
If a Muslim who is not related feels happy to hear the news about this sister being guided and coming back to her religion, then one would expect that her family would be the happiest of people, because her leaving her religion, going away from them and agreeing to live with a kaafir foreigner in an unlawful manner causes a great deal of distress and it is something that many people could not bear. Hence we advise this brother to fear Allah, may He be exalted, and to not be hasty in his decisions and not be a cause of harm to his family or cause division among them. In fact he may be a cause of something that is even worse than that; he may be a cause of his sister going astray a second time. No one knows what ideas of misguidance the Shaytaan may instil in the minds of those who have been guided.
As this woman's father has agreed to let this man who has been guided to Islam marry his daughter, the brother has no right to object to his father and threaten to cut off his sister. Yes, he has the right to express his opinion and he has the right to discuss the matter with his family and his sister. But they do not have to comply with his opinion or accept it.
In this case we say: this brother does not have the right to cut off his sister, because her marrying that one who has been guided is not haraam. What sin has his sister committed by agreeing to marry that man, such that he has the right to cut her off?
Although we say that marriage to that man is permissible according to Islam, and that the brother's objections carry no weight and he has no right to cut off his sister if she agrees to marry him, nevertheless we advise the father and the sister to think again about their agreeing to that man, and they should listen to the other side and discuss the matter with wise people who are close to them and who are aware of the background. It is also essential to investigate this suitor and find out how sincere he is in his Islam before he comes to propose to this sister, and they should find out whether he really is interested in Islam.
We ask Allah to guide everyone to that which is in the best interests of the family and to help them to find the right way and to unite in following it.
And Allah knows best.
Reproduced from Islam QA