Answer
Salam Dear Habsa,
Thank you for your question. We highly appreciate your concern about your sister.
We also appreciate your acknowledgement of the role she played in your life in terms of taking care of your and helping you with your education.
Let’s agree that you sister had taken the wrong decision when she married that Christian man. Muslim women are not allowed to marry non-Muslims. Christians and Jews are not an exception. This is what God (Allah) has decreed and we have to abide by it.
We read in the Quran what means:
{…and do not marry (your girls) to idolaters until they believe…} (Al-Baqarah 2:221)
We also read concerning the immigrant Muslim women:
{Then if you know them to be Believers, do not send them back to the unbelievers. They are not lawful for them (as wives), nor are they lawful for them (as husbands).} (Al-Mumtahanah 60:10)
Your sister has to end this relation with the Christian man at once. She is not allowed to live with him anymore.
The children of your sister are considered Muslims as the children follow the best religion of the parent’s different religions. In this case, Islam is the religion of the children not the religion that the father follows, if any.
No comes your role in correcting the behavior of your sister. First, you and other family members are best advised not to severe your relation with her. Do not wait for her visiting you. Always take the initiative and visit her. If visiting her frequently is difficult, try to call her. Make her feel that she is still a very important member in the family as she used to be when you all were young.
Try to search for a nearby mosque in her neighborhood. Try to introduce her to the Muslim community. This might be difficult in the beginning, but with some efforts you will find some Muslims in her area.
Give your sister and her kids some books about Islam so that they can read them to increase their knowledge about Islam.
Strengthen your relation with your sister’s children. It is very important to win the daughter to your side. Always ask about her and inquire about her progress at school. Explain to her that a Muslim girl is not allowed to date. Islam prohibits premarital relations of any kind.
Tell her that once someone proposes to marry her and she agrees, she can know as much as she can about him within the guidelines of Islam. Tell her not to be deceived by others. Tell her from experience, all stories of dating in the West have ended badly. Islam introduced the institution of marriage to regulate the relation between males and females.
Make a lot of du`aa’ (supplication) to Allah the Almighty to help your sister get out of this dilemma.
We hope this answers your question.
Salam and keep in touch.