Kylie, 17, has shown off much fuller lips of late, sparking reports that she's had surgery to enhance them.
KIM AND KHLOE KARDASHIAN HAVEN'T SPOKEN FOR SIX WEEKS: 'THEY'RE PROBLEMS AREN'T FIXABLE'
But 34-year-old Kim denied this, insisting that her little sister is just really good with make-up.
Speaking to YouTube make-up artists Sam and Nicola Chapman, Kim said: "I go into her room, and I'm like, 'What liner do you wear?' because she only wears a lip liner, and then she fills it in [with the same liner].
"She doesn't wear a lipstick or a gloss."
KHLOE KARDASHIAN'S 'REAL FATHER IS LIONEL RICHIE': STRIKING RESEMBLANCE TO SOFIA RICHIE
Revealing her sister's choice of brand, Kim added: "She goes between [MACs] Soar and Spice [pencils]. I use Stripdown.
"She's so good. She's so good at make-up. Every event, she only does it herself."
Hitting back at the rumours herself earlier this week, Kylie tweeted: "How long are we gonna talk about this lip thing lmao [sic].
"Let's get our lives together guys and talk about some important s***. Just talk about something new at least. I'm bored [sic]."