Why is my little sister who is 3 years younger than me almost the same height as me?

Theres no exact way to tell how tall either of you will be. I think males stop growing at 23, and I'm not too sure about girls, I think its around 17. Personally for me, when I was 13 I was around 5'3-5'4, and now I am 15 and 6'0.5, and I hit my growth spurt when I was 14. My dad is 5'9 (childhood sickness prevented him from growing much more) and my mother is 5'2. Average height for men in America is 5'10.2, and the average height for women in America is 5'4.6. Sorry I didn't answer your question directly, but hopefully this info will help you a bit.