lip
脣 centers on the lip, the fleshy border of the mouth.
脣 combines 月 (flesh) with 辰, which likely contributes the sound. Together they express the fleshy lip.
The flesh radical 月 sits under 辰, like the soft fleshy part of the mouth: the lip.
For シン, imagine a thin lip: thin -> シン, and the lip is the thin fleshy edge of the mouth.