Woman with multiple relations

My step-sister (we have different fathers) has been having multiple relationships at the same time for several years. She lives with her fiance (who is 15 yrs older) and she usually has also a lover or two. Her father diverced with my mother and re-married 2 times since. I am assuming this causes the behavior of my step-sister. What is particularly bothering, she tends to "entertain" the group of our friends with the stories how she has yet another lover (who is also married) and she still lives with her fiance. I would like to help her see how little respect she commands from her "friends" for this and would like to help her. Can you please advice?

Tell her about your feelings about her behavior. She is probably not aware of that her friends are just "friends".