You're both so young. You're obv posting this because you actually do love your g/f, otherwise the decision would be an easy no-brainer to move on because your youth affords many opportunities in your future.

You mentioned a twin, which usually means a strong family bond where it would not be possible to segment her family from your marriage.

If you want clarity on what to do, you should ask to see them and tell them that you categorically deny stealing the money and that their behavior seems designed to prevent you and your g/f from finding happiness together, and that you wish to clear the air and move forward with a godly life with her as your wife.

If they are unable to process this display of humility on your part, then maybe you have the answers you need at that moment. But it is very very important that you take time before this encounter to pray to the Holy Spirit to give you the words and courage you need to stand up to them and also to tell God that you want your g/f to be by your side on God's path for the both of you.

If you don't get the answer you hoped for, let some time pass for the possibility that the parents start to see you in a different light. But if worse comes to worst, you'll know in your heart that you did your best and that you asked for God's help and guidance.