After all, their real selves just sleep all day and their families don’t seem to pay attention. I know Japanese families are at large dysfunctional. But you would have to be completely oblivious not to notice extra people turning up.
Well, given that their replicas leave the bedroom, have breakfast, go to school, etc. there's no reason for the parents to be aware of it if both parents are at work. Unless they happened to be home during school hours,
and they happened to go into the kid's bedroom, they might never know. I expect most parents wouldn't know anything was wrong unless the school contacted them about absences.