suck, sip
啜 centers on drawing liquid into the mouth in small amounts: sucking or sipping, often with a soft sound.
啜 combines 口 (mouth) with 叕, which likely contributes the sound and may relate to a repeated or soft action, suiting the idea of sipping or sucking.
The mouth 口 on the left and the repeated shape 叕 on the right suggest a mouth making small, repeated sucking motions: sipping or sucking.
For セツ, imagine sipping a hot soup and making a soft 'sets' sound as you suck it in: sets -> セツ.