Of course this is natural behavior! Can't you read? Pick up any book or article on the topic and you will see that this is very normal.
Children that are embarrassed or made to feel shame for exploring grow up to be sexually inhibited adults!
You should talk with both children and let them know that it DOES feel good and that it is perfectly fine to touch themselves. Tell them that it is very inappropriate to touch each other as they are brother and sister. Let them know that they should never touch themselves with anything other than their hands because they could hurt themselves.
Leave it at that and if they continue with each other, have another talk with them and let them know that it cannot be accepted but that you still love them. Tell them what they are feeling is natural and loving but not to touch each other.
Relax mom....you don't need to break your family up over this. Pick up a book to learn more.
