I have a theory that your siblings determine how tidy you are in your adult life, but I am not exactly sure how it all works out.
My theory is based on public goods and free-riding. If as a kid you shared a room then you and your sibling didn’t internalize the full marginal social value of your efforts at tidying up and as a consequence your room was probably a mess. At least messier than it would have been if you had the room to yourself.
This would suggest that if you want to know whether your girlfriend is going to be a tidy roommate when you shack up one easy clue is whether she has a sister. If not then she probably had a room to herself and she is probably accustomed to tidiness.
But here is where I start to think it can go the other way. A kid who shares a room needs to adapt to the free-rider problem. It pays off if she can develop a tit-for-tat strategy with her sibling to maintain incentives for mutual tidiness. This kind of behavioral response is most credible when it stems from an innate preference for cleanliness. Bottom line, it can be optimal for a room-sharing sibling to become more fussy about a clean room.
As I said I am not sure how it all balances out. But I have a few data points. I have two brothers and we were all slobs as kids but now I am very tidy. My wife has no sisters and if I put enough negatives in this sentence then when she reads this it will be hard for her to figure out how untidy I am herein denying she never fails not to be.
Her brothers also cannot be accused of coming dangerously close to godliness either but I don’t think they shared a room much as kids. One of them now lives in an enviably tidy home but I credit that to his wife who I believe grew up with two sisters.
My son has his own room and at age 5 he is already the cleanest person in our house. He is also the best dressed so there may be something more going on there. My two daughters have been known to occasionally tunnel through the pile of laundry on their (shared) floor just to remind themselves of the color of their carpet.
I have a cousin whom I once predicted would eventually check herself into a padded cell mainly because those things are impeccably tidy. She always had her own room as a kid. Sandeep is an interesting case because as far as I know he has no brothers and while his home sparkles (at least whenever they are having guests) his office is appalling.