I'm 17 and I'm 5'8". My little sister is 15 and has already outgrown me by 3 inches. I don't understand it, it makes no sense, and it embarrasses me no end. A sister is not supposed to be taller than her brother and being it's my little sister makes it even worse for some reason. She is now the tallest in my family.
Boys grow till about 21 years old and girls till about 16 years old. You will probably be taller than her. Generally boys end up being taller than both parents and girls end up being taller than the mother but not as tall as the father. But of course there are exceptions as well. She might just have had a (final) growth spurt and you are lagging behind, but don't fret it will probably change still. :-)