Riddle from Riddle me itzy (Late Summer riddle)

It was a hot day, the sisters Yuna and Ryujin were out camping at the woods along with their friends Chaeryeong, Lia and Yeji. They planned to do fishing,swimming,hiking,etc. When they got there. They found a spot nearby the lake. After a long right. Yuna needs to use the washroom. Luckily, there was a huge lavatory nearby their campsite area where more people can go in!

"I gotta use the restroom nearby" Yuna said to her sister and her friends.

After 7 minutes later, Yuna was finally finished! So, she went back to campsite and found her sister, Ryujin...DEAD with a knife on her back! She couldn't the police because there were no signal. so she talked to her friends. Chaeryeong, Lia and Yeji to see who looks suspicious with her question. "What were you doing while I was gone?"

CHAERYEONG

"Sorry that I left for a while it's because I got impatient waiting for you to help me set up the tents. So, I was fishing by the near lake since I bought a fishing rod with me, I asked Lia and Yeji if they wanna come fishing with me but they said "No!" so I went all by myself. It was fun but sadly, there weren't any fish in there. Well, I gotta wait til next tomorrow."

Lia

" I packed alot of snacks with us. I hadn't ate for a while. So, I went in my car to get some snacks. When I returned. I was really tired of getting some snacks but luckily, Ryujin was there to hang out with me and she told needed to use the restroom...So, I waited for her patiently."

Yeji

"I'm impatient to wait for the tent to be set up. So, I asked Ryujin, Lia and Chaeryeong if they wanna look around. Ryujin decided to come explore while the others said no. Basically I was exploring this area for a bit with Ryujin but after a couple seconds, she disappeared. I got scared then I decided to head back to our campsite where Ryujin might be in."

Who do you think did it?