What can i do to show my gf she shouldn't be jealous?

My gf and i are nineteen (almost twenty) and have been dating off and on for about four years and are thinking about marriage. I am in a gospel trio that sings at local churches which consists of myself, my best friend (also nineteen), and his sixteen yr old sister. The three of us are close and I feel as if they are my brother and sister. My gf is rly jealous of my bff's sis and doesn't like me being around her. What can i do to show my gf that there is no reason to be jealous?