Yeah, it can be confusing
The nation of Israel foreshadowed and came with the promise of Christ. The Church, however, is a part of the fulfillment of the promise, through Christ; so the Church is the new Israel. So, the country of Israel today can be understood in two ways:
1. They are chosen by God in the sense that they were given the Torah and the promise of the Messiah.
2. They are not any more chosen than any other country or tribe, for Israel is not understood as a physical country, but a body of believers in Jesus Christ.
Another factor of Israel as a chosen nation has to do with God's self-revelation. It's the amazing and incomprehensible act of God, whereby He revealed himself in humility, born a lowly man of a lowly tribe.
Sin is very difficult to understand on account of it being a spiritual matter and not practical. In other words, I don't think anyone can appreciate the gravity of sin from a purely academic point of view.
However, sin is crucial to everyone, whether we understand it or not. To sin means to willingly rebel against God; to do evil. It may not seem like a big deal, even for Christians, but in reality it's heaping abuse on things we don't understand. Sin is a very severe offense, worthy of a very severe punishment. This ultimate punishment is an eternal separation from God. That is, an existence entirely void of God, who is the source of love, goodness, kindess, mercy, patience and holiness. However, the good news is that believing in Jesus Christ, there will be life and no condemnation. Again, life in Christ is a free gift, by God's grace, apprehended by faith.
This is very wordy, sorry, but I feel it's important to make an effort to establish the severity of sin, in order to get a better understanding of your situation (from the Christian perspective). I think, perhaps, the root of the issue the church is having with your girlfriend, is not so much about whether there is sin or not, but about her (and your) spiritual welfare. The concern is not out of malice or self-righteousnesses, but love. Also, practically speaking, it's not always easy being together with someone when your world views are radically different, and there are many implications that comes out of that, but it's not really for me to judge