It seems that you share a very special relationship with your sister and that she means the world to you. It is understandable that right now, you are not only feeling insecure about your relationship with your sister, but also very concerned about her well-being.
However, one needs to remember that your sister is an adult and has a right to take her own decisions. Instead of getting worried about her, a better idea might be to sit down and talk with her. Maybe if you get her perspective on the matter, you will be able to accept it or make her see the flip side of it. We keep forming new relationships in our lives. A new relationship does not mean that the older ones become less significant. Each relationship has its own meaning and fulfills a different need. Just because your sister has someone in her life, it does not mean that she has been stolen away from you. Still if you are feeling threatened, not sharing your feeling with her is only going to complicate matters and increase the distress for you.