Girls, I have such a strange question. They say you shouldn't show you…

Girls, I have such a strange question. They say you shouldn't show yourself in the mirror to babies. Like something mystical they see there?! Our mother-in-law says we should close all mirrors before bed, and we have a large mirror on our dressing table, so we have to cover it with a sheet. How awkward and ugly ? Does anyone else do this? ??