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This is probably the wrong forum for this questions, but I'm curious on what responses I will get.
You're dating a girl for a long while (months - years). You live with her, you spend your time with her, you know her family and she knows yours. One day however, you become close to her sister, and find out that her sister is the one you really want to be with. The question is.... can you pull the switch? Is it possible? How would you do it? What would you do about the family situation?