I see a lot of RED FLAGS here...
Dear mom,
There are more issues here than I can possibly address. I read all of the threads following your plea for help and none of them seemed to see a real problem here. Embarrassment clearly is not the answer. There are some serious boundary issues going on here. This is clearly INAPPROPRIATE behavior going on between two kids who are plenty old enough to know better. If they do not know better they should at least be able to sense that something isn't right. I don't want to scare you too badly but you should seriously question whether both of these children have been subjected to sexual abuse. It is a horrible and hideous thing to even consider but I assure you that this experimentation between brother and sister is clearly not what most people would consider to be normal. I would suggest that you talk to each one of then separately about this behavior and sexuality in general. Let them know that it is not okay to behave that way and ask them to be honest in their answers. If you can't bring yourself to address this issue with them them please seek out a counselor in the school system or social service system to help you and your children. It is not abnormal for the kids to be curious and many even say that masturbation is perfectly normal at this particular age range but nowhere will you find that it is normal or okay for a thirteen year old girl to experiment on or with her brother or vice-versa. I am by no means trying to embarrass you or offend you in any way. If this is simple a case of misdirected curiosity, it is your job to help the kids set up boundaries, especially when it comes to their bodies.
My prayers are with you and the kids,
Amy