I feel like my fiances sister is getting inbetween us?

Well,,,Now you "know" That when you marry him,,you will be marring her also, You "MUST" remember that blood is thicker than water, Do not fight with Her,, or about her, "YOU HAVE LOST HIM" Just buckle up an move on to someone else, It will be hard,,but thats your only answer,,Do not let yourself get "TIED"into a bad,,unhappy marrige, Just tell him that you are sorry,,Don't argue with him, Don't try to make him choose between you and her, because you will loose. ( You have already lost) Just move on with your life.