Kylie Jenner felt so "sad" over Kendall Jenner's acne struggles.

Kendall and Kylie Jenner

Kendall and Kylie Jenner

The 'Keeping Up With The Kardashians' star knew how much her sister was struggling with her skin condition during their time at school and teared up as she admitted she was devastated for her older sibling.

Kylie said: "I knew it was your biggest insecurity and every time like clockwork we would get out of the car [at school], you would pull me into the staircase and you would say, 'How do I look today?' ... It always made me so sad to even hear you talk about it. I'd be like, 'You're so beautiful,' trying to help. But then I had to go away by myself. It makes me feel bad. When you have your own daughter you just think about this."

And Kendall admits she started wearing makeup from the start of 9th grade to cover her acne.

Speaking in their joint YouTube video, Kendall explained: "I remember the summer before 9th grade is when all my acne went crazy and I was like, 'I'm going into high school and this one guy's really hot.' I'm going to ruin all my chances! I didn't! We ended up dating. It was the confidence that came from within that really probably turned him on to me ... I would have so much acne. Every morning we would show up to school. Dad would drop us off and we'd be walking up the stairs. I'd look at you and I'd be like, 'Can you see my acne?' Every day you would say, 'No you look beautiful.' I would be like, 'Okay.' I knew you were lying but I appreciated you."