Q:

Is it idiomatic to say "please always close the door behind you"?

my sister's bedroom is face to my bedroom and she play music loudly, which is disturbing me. So, I said to her, please always close the door behind you. I would like to have her remember this, and close the door next time without my reminder, close our own doors to avoid disturbing each other.
Is it an idiomatic expression?

A:

"Always" may sound more commanding than you want and actually if you are not her boss, it may not be justifiable to say that.
"Behind you" is redundant when it's obvious, so you end up with "Close the door, please."
If she's a decent human being, she'll understand even better if you give here some reasoning: I try to study (sleep, or so), and your music is disturbing me, so would you...? If you have a good reason, you'll find the right words.

