It is and it isn't all in the same breath- At 9 their really isnt an "average" height, because half the girls are well into puberty, and the other half are nowhere near it, this effects height, because one of the first symptoms of puberty is a massive growth spurt in females, some can grow up to 6 inches in one year when they enter adolescence.
Its clear as day your little sister is well into puberty, I would even venture she is over 50% complete it. She's hit puberty a bit early, the average age is 10, so give or take shes a year early, it's common. Once she has her first period, she wont grow much more, at most she'll gain another .5 to an inch. Lets say she has period a year from now, she'll gain 3 inches in a years span which will put her at 5'5, and after she has her period she'll level up to 5'6 for her adult height.