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But if you and say his sister are best friends already he will just have to suck it up. There's no real definitive rules on it. I know my parents and older sister kept my 2n eldest brother's ex close long after they broke up because they liked her and she was good for him and they really hoped he would Marry her rather than the train wreck he eventually did. Funny thing though she ended up with my older brother after he left the marines and are happily married now with kids.