Could this be what she thinks?

Good day house, please I need help. My sister is in what she describes as a serious relationship with a man who says he wants to spend the rest of his life with her. My sister is 35, her man is about 40. Her boyfriend travels a lot for work and she has told him that because of this she is going to be continuing to have s.x with various other men. She has friend with benefit relationships now with 3 married men. She also has oral s,x with men she meets out and about in the street or through the course of work or normal social events. We are not really close so I have not yet asked her if all is OK or why she is doing this. I am really scared for her because she is acting like someone that has a disease. Please does this sound like s.x addiction?