Hey.. According to Bible, we all are siblings. We all came from Adam and Eve. We are the children of them. If your families agrees with you two, its not a sin. Moreover, it may be God's plan. But if they disagree, you two should accept it because (according to the bible), if we go against parents will, its no different from going against God's will. The main thing is just put it in God's hand. Do not consider is as sin. Just consider it as lesson for your life. JUST PUT IT IN GOD'S HAND. God will show you the way. You will feel better after you leave it to God. God bless you!