Lyric discussion by Icaus 

Cover art for Sisters lyrics by Pain of Salvation

I think this song is about someone ''wanting'' the sister of his girlfriend.

the song mention three persons, and the second paragraph he is comparing two persons, his girlfriend talking of her as ''you'' and his girlfriend's sister talking of her as ''she''

And she walks like you And she smiles almost like you A child of the wild just like you

he is saying that his girlfriend's sister looks like his girlfriend

And perhaps it's the want Of (other) you in her eyes But I want her this one single time ...Just this one time

this second paragraph tells that he wants his girlfriend's sister

tho both know whats going on on both minds (''Both so awkward, for what's on our mind''), they both know the consequences so they say ''good night'' to don't make the big mistake of ruining a life they both know ''his girlfriend life'', sleeping with you girlfriend's sister would ruin her.