The problem with the scared-of-being-hurt-no-longer-try theory is that our interests and needs have their own individual weights and if we want something, it does no good to give the "sour grapes" response and decide you aren't going to try. I did not get married until I was 39 years old but I obviously didn't stop trying even though it was a difficult process for me?

Your sister does not want to be in a relationship and married right now; that is all that can be known. We cannot know how we will feel in the future, what will happen and how it might affect us. It took me a really long time but I quit saying, "I would never do. . ." because, inevitably, the situation would rise where I had the opportunity and interest in doing whatever it was. If one has not had an experience, one cannot know how one would feel in that experience. It is like not liking a food you have never tried.

I would just accept and respect your sister's current interest in not marrying but not worry too much about how she will "turn out" in the future; she will or will not marry but it probably does not have a whole lot to do with a declaration made before she meets someone she would like to be with the rest of her life. I don't know anyone who meets someone they want to know or be with who says, "Sorry, back in 2011 I swore I did not want to ever hook up with anyone so I'm going to have to leave now. . ."