6 yr. old son curious of private areas on 3yr. old daughter

My 6 year old son seems to be curious of my daughters private areas and his.  I am very protective of my daughter (because of my own issues), and I am worried I am not handling this situation sensitively enough.  I get angry and yell at my son when he spanks my daughters behind and seems to purposefully place his hands on her privates when they are playing.  Now I do not get a creepy, strange feeling when I witness this, I do feel this is innocent curiousity.  I am just scared it will go to far.  I feel I should be the one to help him through this, but when I try to talk to him privately he has no questions for me and does not say much.  
My daughter thinks it is funny and she adores her older brother.

[Kevin Kennedy, Ph.D.]

Your son should not be spanking his sister nor touching her genital area. It's important that you set firm limits on this. In addition, you can teach him about the differences (bilogically) between boys and girls, and I suggest that you use children's literature to help you with this. Don't expect him to have questions for you. Ask your local librarian for resources and he/she can help you to select books that are suited to your son's age.