Why are sometimes skinny people (those that don't even have muscles) heavier than fatter people?

I'm guessing your sister is not being truthful about her weight. I'm 5'3", and the short time I was thin in high school, I weighed 120. So I have doubts as to whether someone with thick thighs and big hips really weighs 110.

Have you ever SEEN your friend on the scale? 115 is a healthy weight for 5'4, and if you are saying she's super skinny, it might just be a cover so you guys don't worry about her. OR... your sister is the liar. have you see her on the scale? I'm not saying it is/being accusatory oh & just keep an eye on your friend 'cause I didn't even know one of my (ex?) BFFs smokes weed for like a long time after she started grr weed is nasty not that I've ever tried it we are hardly even friends anymore though

It would depend upon the bone mass too people with thicker and heavier bones will have more weight though being skinnyor looking like that

your skinny friend still has muscles and it might make her heavier