Is it normal my sister got her friend to flash me?

This was in my last year of high school but I still remember it. I was doing a group assignment and one of the people in my group was my sister's friend. I was doing most of the work. So I was at her house to talk about I had finished the assignment and my sister was also there just hanging out with her friend. I was going to go but my sis said to her friend like he put a lot of work into this for you so you should at least flash him or something. And her friend said fine and did. And I remember my sister kept smiling at me until she stopped. It was so weird.