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Here’s my suggestion about house rules: if your adult sibling came to stay with you for a few months, what house rules would that person be expected to observe? I think this can clarify whether something is really a house rule, or if it’s really a vestige of the exertion of parental authority.
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Good question!
If a sibling came to stay for a few months and we had a “no eating in the living room” rule, I’d expect them to honor it (we don’t have rules like that, but some people do).
I wouldn’t allow a sibling to play blaring music in the middle of the night, or drive our cars w/o asking first.
If a sibling was being a total couch potato - day after day - then I might gently try to explore what was behind that. If there was nothing medical/mental going on, then I’d try to brainstorm some outside activities (sightseeing, whatever) to help get them out in the fresh air occasionally.