I have a sister dating a non-Christian for over five years now. She have been teaching him, but he still haven't yet make a decision to get closer to Christ. She is deciding that in the next few years if he does not change to not get marry entirely. Honestly I do not think it is the best idea. What can I say to encourage her?
Your sister needs to realize that this guy is not interested in the Gospel, only her. I agree that she should move on and not marry him. I have known way too many Christians who took spouses who were not interested in God, and it almost always ends in heartache for the Christian. They get no support when the children come and don't want to study their Bible lessons, and often the spouse keeps them from attending worship for worldly pursuits. Many times the spouses get jealous of the time the Christian spends at worship and in fellowship with other Christians, and ask them to stop going or not go as much. Children see this conflict too.
Tell your sister to spend the next few years looking for a good Christian man who has the same values as she does. I'm sure that with the same effort toward prayer for a proper spouse, she will not need many years.
Look at the example of Solomon in the Bible. When he took foreign wives, they drew him away from God. "But King Solomon loved many foreign women, as well as the daughter of Pharaoh: women of the Moabites, Ammonites, Edomites, Sidonians, and Hittites- from the nations of whom the LORD had said to the children of Israel, "You shall not intermarry with them, nor they with you. Surely they will turn away your hearts after their gods." Solomon clung to these in love. And he had seven hundred wives, princesses, and three hundred concubines; and his wives turned away his heart" (I Kings 11:1-3). This happened to Samson also.
Best wishes to you in convincing your sister to search for a Christian man.
Thank you for your words of encouragement.