OK, I had something happen like this when I was a child. Not with a brother, but with a cousin. It is important that you have a sit down with both children. Both kids need to know what they were doing was not appropriate. It is understandable as a mom myself, I may have freaked out at a situation like this too. The five year old needs to know good touch, bad touch and how to protect the gift that God has given her. I'm not quite sure how to do this with such a little one. Maybe a counselor could help you with some good advise. The son needs to know you love him and he needs to have a talk about sex. I think it is most natural coming from the father, but my son asks me all his questions and I am very open and honest with him. I will answer any and all questions from him. He needs to know it is natural to be curious, but this is something that you only do with you beloved husband or wife. And when you wait for that one person it will be wonderful and is such a gift to share with them. I would pray with him and really let him know he is a good boy that didn't know better, and he does need to ask Gods forgiveness. We all do when we go past our boundaries. If he doesn't get forgiveness, he will have a hard time forgiving himself one day.