I think that this is actually a very serious situation. You boy is 13 and should know better. At 7 years older then his sister he has the control in their brother/sister relationship so she is likely to do what he tells her as to her he has authority.
I am actually a victim of childhood sexual abuse in a situation that wasn't that different from this. What I would definately say is that if you do not take this extremely seriously your daughter will grow up feeling completely abandoned by you. You are the one who has to stop this and deal with it and I know that is incredibly hard as it is your son who has done it, but the fact is that he has touch her privates and as he is old enough to know better (he doesn't have to be an adult) then that makes it a sexual crime.
I'm really sorry that this has happened to your family and I'm sorry if what I am writing makes your feel worse.
I agree that your daughter should see a doctor (she is probably fine but you should still get her looked at). She is also likely to need counselling. She needs to talk to a professional about what has happened and why and also to prevent her from copying what he has been done to her on another child, not saying she will but just in case, as children that young do learn from what they are taught by their elders and your son has taught her that touching siblings is fine.
Your son obviously also needs a great deal of help to find out why he did it (has someone done this to him?) to prevent him doing it again and to stop this from being something which continues throughout his life.
I know this is terrifying but as a child who had to just put up with it and no one know or did anything, you really do have to take action and a "stern talking to" as one person suggested really doesn't cut the mustard.
I don't actually know what is the best action. NSPCC would be a good idea or GP or social services.
I'm so sorry for you all :(