I said to God that I would fast for a day for my sister (to bring her to Jesus) but I ate about 2 hours ago, I broke the promise. I didn't even realize it was a sin but God made me realize how important it is to fulfill my promise, I read that it's better even not to make any vows to God. I'm a baby christian but that's no excuse, I feel terrible. My question is, will God still guide my sister to repentance even if I failed to finish my fast??? Now I know I should guard my mouth and be careful not to make vows that I cannot keep, and I'm so grateful that God made me realize this, but I ray to God that He will still help my sister, to soften her heart and guide her to repentance like He has done for me.
It's not fully correct to say that we fast (or pray) to get God to do something. Prayer and fasting are ways for us to participate in what God is doing (2 Corinthians 1:10-11), however we can move his heart according to what we desire, but it's up to him to decide what to do with your prayers and fasting. The closer we are to God the more the things we naturally want will be in line with what God wants, and the more often we will see our prayers answered (and fasting accomplishes more).
By not keeping your promise to the Lord, you sinned, and the consequences on earth are happening. But your relationship with the Lord was not harmed much because you didn't know it was a sin. If you confess what you did as a sin, it will show the Lord that you take what you did seriously (which is actually for your benefit, not the Lord's).
It's really up to the Lord what to do with the fasting you did do and with the breaking of the promise. One cannot say that because you did ABC, God will or won't do DEF. However, are you really sorry you broke your fast? Do you want to avoid diminishing the reliability of what you tell the Lord? Do you
really, really want your sister saved? I urge you to fulfill your promise by fasting again for the full amount of time. (Drinking lots of water makes it physically easier, as does keeping yourself busy, instead of pondering how much you want some food. Asking the Lord for strength while fasting also makes it easier.)
It is valuable to recognize that God already wants to save your sister, so you know that you are praying in his will when you ask him to save her. However, recognize that it is a spiritual war to turn someone
else's heart to believe in the Lord and the yielding and repentance that leads to Life. Praying and fasting advances the battle in the direction that God wants and what you pray for, but sometimes it is just one step in a long war. Gird yourself with the full armor of God for a protracted battle (Ephesians 6:10-18) and pray and fast regularly for the salvation of your sister. God will guide you; remember he is the one doing the saving.
(Off topic: I recommend the book,
My Utmost for His Highest, for anyone. It is a compilation of Oswald Chamber's teachings by his wife after his death.) One thing he said was,
Prayer is not a preparation for the battle; it is the battle! (which is true, although it can also be preparation for battle)
Note that Jesus taught that we must not make oaths/vows (Matthew 5:33-37). One of the reasons is that we do not know what our future circumstances will be in which we must keep our oaths. (However, for those that made oaths, they must keep them.) Instead just set in your heart what you determine to do and do it. In the context of doing what you determine to do, it very well may apply that if you just tell God you are going to do something, that it counts as an oath. Rather tell the Lord you would like to do something or are going to
try to do something—and ask for his help. (Matthew 5:37; also see James 4:13-15)