Kids are growing up much faster today then when we were kids. With the internet, face book my space etc etc kids have much more information at hand then 10 years ago. Schools,the library, friends all have computers and kids know how to use them. By 10 they are talking with each other about boys and sex and the changes that are starting to happen to them. They get curious and go surfing to see what they can discover.
Masturbation isn't evil or harmful. If she has just discovered what it does then she will enjoy doing it as often as possible until it becomes old to her.
Talk to her and explain the facts of life to her. Be honest and up front with her. Be open to her questions and answer them as best as you can. If you don't have an answer go on line with her and find the answer.
She"ll learn that masturbating is not the only thing in the world to do. If you treat it as normal and just go with the flow she will slow down and be come more private about as she gets older.
As for her sister she most likely will learn from her sister but no harm done. Just be open and honest and let both of them know you are there for them . Its part of growing up believe me, I have 3 daughters and a son all adults and all healthy and well adjusted.
I think its us parents that get the shock and the nerves when it comes to talking about sex. However if you don't teach them at home they will learn it on the street. The question is where do you want them to learn it. Good luck and god bless