What is the normal weight for 12 years old ???? For my little sister

Question

Answer

Depends of her hight. My daughter is 98% in weight and 100 % in height. When you combine the two then she is just at 50%. So that means that from a hundred of kids her age that have her hight she has 50 kids that weigh more than her. 
  Most kids and parents don't take in account this thing. I weigh the same with my friend. She is super skinny and tall (looks at least 20kg less than me!). I am average height but have big bones and I am curvy. No one would believe we have the same weight even though we are the same age.
  My point is you can't get an accurate answer to your question. The best thing your sister could do is ask her doctor what is her procentage   or find a website where she can input her hight and weight to find and accurate answer.

Answer

I imputed her hight and at weight I put 50 kg so you guys can see what I mean. Like I said your question was not accurate because you put wrote jus her age and not her hight.