Is it ok to be 5'7" and weigh between 90-100lbs? if you are 15?

Yes: It is my opinion that if you are eating healthy and exercising regularly that for the time being this weight is ok. It is important that you are not taking any steps to avoid gaining weight as an ideal body weight would be closer to 135 by the time you are fully grown i.e. age 21. Keep in mind that many athletes and younger people may not fit into a chart. The key is to be healthy.

Yes and no: By the numbers you are now at about 90%ile for age by ht and 10%ile for wt. A tall slender young lady. Most women grow their height for about 2 years past their first menses. Ask your mom if she was tall and slender at 15 also. Have you been anorexic or bulemic? Talk to your doc.

YOUR BODY PROPORTION: ...ARE DISCORDANTS TO THE POINT OF CONCERN FROM A PEDIATRICS STANDPOINT...YOUR WEIGHT FOR HOW TALL YOU ARE SHOULD BE CLOSE TO 150 POUNDS. IF YOU ARE HAVING NO PERIODS, OR PROBLEMS WITH YOUR SKIN, HAIR OR NAILS IT IS VERY IMPORTANT TO GO VISIT YOUR DOCTOR! GOOD LUCK