I think maybe your T missed and opportunity to explore something interesting with you. As I see it the issue isn't so much whether open marriage is okay or not but rather what your sister and her choices and her marriage mean to you. What did it mean for you when they were (in your eyes) the perfect couple? What hopes did it give you about your future? How/why is it important that she be different from your parents? What are your feelings and experiences about monogamy? There are about a million questions in this vein that I think would be more fruitful for therapy than a discussion about the relative merits of somebody else's open marriage. In my opinion.