Does the brother have the right to object to his sister’s marriage and to threaten to cut her off when her father agrees to it?
Question

                There is a Muslim woman who was married to a Muslim man and they had a child, then soon after that the Shaytan caused her to slip, and she started to go to nightclubs where she got to know a kaafir man and a relationship developed. Then she took her son and went to live with that man unlawfully, after leaving her religion and giving up Islam. Then a year after that, she repented and left her boyfriend; she entered Islam again and began to wear Hijab and pray, and she mended her ways completely. She went back to her parents and asked them to forgive her, and they forgave her. 

Then a few years after that, that boyfriend became Muslim and he came to her house and asked her father for her hand in marriage, and the father agreed, as did she. But her brother did not agree, and he was very upset to see the man who had committed immoral actions with his sister, and for whose sake she had left Islam and abandoned her husband. It upset him to see him as his sister’s husband, so he did not agree to the marriage at all and he threatened to cut off all ties with his sister if she married him, and said that he would never speak to her again after that day. 

This behaviour upset his parents greatly; they do not want to see their children fighting. Hence the father tried to remind his son of the rights of kinship and tell him that it is not permissible to sever those ties and that Allah has enjoined upholding those ties. 
But the son refused to accept that and he said that it is within his rights to cut her off and that this is a right guaranteed by Islam. 

My question is: is this a right that is guaranteed by Islam?.
Praise be to Allah.
    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.