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I don't know the LW or her boyfriend or his parents so they can decide for themselves if this would work in their case, but one thing I've found in my own experience with managing overprotective parents is that if you give them a great big sit-down "Mom, Dad, we've got something to tell you," that makes them think they get some say in the matter. If you just mention it in passing, that tells them that it's a done deal and not up for discussion. So instead of a great big talk, just stop hiding and tell the truth when it comes up. If they ask, just say "Oh, she lives here now," then move on to more interesting topics.