Yes, physical description relates to show and tell. Everything on this site is related. However, creating Topics and limiting offshoots of discussion within each Topic makes searching for and understanding specific information easier.I know when I'm looking for answers on this site, I'm more likely to read an older thread in its entirety if the discussion stayed more or less on-topic, because it's more difficult to see relevant keywords in a jumbled discussion.
Then again, the title of this topic implies we're talking about imagination rather than the definitions of "show" and "tell". Maybe my mallet is striking a brick wall.
dougsguitar, would description of your characters provide any insight into their personalities? That is, does the sister notice her younger brother is gaining muscle mass and wish for the days when he was just a scrawny but adorable kid? Does the brother hate his blonde curls (or whatever) because everyone else in his family (for example) has straight brown hair? Don't break the flow of a story or change perspective to provide description. Consider how these characters see themselves and each other, then look for moments when their views of physical traits might influence actions (for example, the sister stifles her brother or the brother tries too hard to fit his image of his family).