Her Brother Lived with Christian Lady and Now Wants to Marry a Muslim
17-9-2002 | IslamWeb
Question:
I am a Muslim lady. My brother lived with a Christian lady without marrying her for eight years. Now they have left each other. My brother wants to marry any Muslim lady. My parents have chosen a religious Muslim girl and my brother is very happy. Is this a right thing to do according to 'Surah An-Nur', as these days many people do this? My brother is not very religious; he sometimes offers prayer. Please advise me what I should do.
Answer:
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.
Zina (fornication or adultery) is a major sin for which Allah, The Exalted, threatens with severe punishment.
Allah Says (what means): {And those who do not invoke with Allah another deity or kill the soul which Allah has forbidden [to be killed], except by right, and do not commit unlawful sexual intercourse. And whoever should do that will meet a penalty. Multiplied for him is the punishment on the Day of Resurrection, and he will abide therein humiliated - Except for those who repent, believe and do righteous work. For them Allah will replace their evil deeds with good. And ever is Allah Forgiving and Merciful.} [Quran 25: 68-70]
Thus, it is a duty for one who commits such a major sin to repent at once to save himself from destruction. Therefore, you should advise your brother strongly and remind him of what Allah has prepared for those who commit this indecent deed, hoping that Allah will guide him to the right path.
As for giving up the prayer, it is for sure a greater sin, since the Prophet's Companions did not consider the abandonment of any act as disbelief except abandoning the prayer.
The Prophet said: “The pact that is between us and them (non-Muslims) is the prayer; whoever abandons it is a disbeliever.”
So, it is strongly advised to remind your brother with the bad consequences of his present situation.
As for marriage between one who commits Zina and a chaste woman, it is permitted with reprehensibility, according to the majority of Muslim scholars.
As for the verse in Surah An-Noor which means {The fornicator does not marry except a [female] fornicator or polytheist.}, it means that Zina will be taken place only between a Zani (one who commits the Zina) and a Zaniya (a female who commits Zina) or a polytheist like him or her. But, a Muslim chaste woman or man does not accept such an action.
Imam Ahmad 
believed in the impermissibility of marriage between a Zaniya and a chaste man, and vice versa. He deduced his opinion from the above-stated verse, but the opinion of the majority of the scholars is the dominant one.
If your brother persists in his disobedience and immorality, then it becomes necessary to advise the fiancée to refuse him if she is a pious and chaste woman.The Prophet said: “(the basis of) Religion is (sincere) Advice.” [Muslim]
However, it is your duty to choose the best means to advise him in a soft and gentle manner and at an appropriate time and place so as to avoid causing a greater harm, such as the cutting of kinship relations between you and your brother.
Allah knows best.