Friend's Little Sister is Pregnant at 13!?

Okay so my good friend, we will call her Jess, has a younger sister. We will call her Kayla. Jess and I are 17, almost 18. Kayla is 13. Recently, Kayla has been acting strangely and stealing Jess' clothes. They are clearly too big for her. Earlier today I walked in on Jess in the bathroom because I didnt know anyone was in there. She was in a bra and pants, and her belly was CLEARLY rounded and poking out. You could tell she was about four months pregnant, she's very thin so the belly wasn't too big but it was obvious. She freaked out and begged me not to tell but I am torn. She is only 13 years old, and looks about ten. She is underdeveloped and barely five feet tall, I don't even think her body can handle making a baby. She has also looked very sickly lately. Kayla is like a little sister to me. I never ever thought she would get herself into a situation like this especially at 13. Should I tell Jess and her parents or what? Kayla already said she's not going to tell until she like goes into labor. I just know that she needs better care, vitamins, something. But I don't want to hurt her. I just want her to be okay /: help!