DEAR ANNIE: I’ve been in a relationship for 10 years, and we got engaged this year. The issue I’m struggling with is that before we were official, I noticed a message on social media of my groom in which he was flirting with my best friend’s little sister. Now, 10 years later, we are still often at get-togethers with her, and I sense there is some flirtation between them every now and then. At times, this really bothers me. Is it because I know he was interested in her years ago? Should I mention this to my future husband, or am I being immature? — Bride-to-Be DEAR BRIDE-TO-BE: Based on the f…