At 13 some kids are children and some kids are little adults, and some are waffling inbetween. I wish they were all waffling.
The first thing you need to find out is was this the first time this happened - or is it just the first time you found out.
If it is the first time - or there have been only a few times. I can help.
If this has been going on for longer than six months, or is happening frequently. You need to get professional help.
So the first thing you need to do if you have not already done it is to determine exactly what happened. Was it the first time? Was it OK with both of them that it was happening?
If it was the first time they may have been unlucky that you discovered them. Many siblings experiment with sex. It isn't something you want to encourage, but it is not something that means your kids are scarred for life either. If they were just checking out their bodies and seeing what it was like to kiss- then a simple discussion about sexual boundaries between siblings may be all you need.
Be clear that sex is not bad- but people who are related to one another do not have sex with each other for many reasons. They cannot marry. They cannot have children- and if they do their children may not be very healthy. Also let them know that they risk having one or both of them being remnoved from your house by child welfare if anyone found out.
Then you need to have the discussion every parent needs to have with their kids of this age about what you value about relationships and sexuality and encourage them to behave in a way that you approve of. You cannot control what they do - so you can only encourage them to do what you think is best.
Now is the time to be sure both of them understand the risks inherent in having sex. No matter what birth control method or methods you use - there is always a risk of pregnancy. So do not have sex unless you are prepared for the possible consequences.
This could be one innocent time of experimentation. Or it could be they have done it a few times and they are exploring. If that is the case you can be firm about the time your discovered them being the last time. And as difficult as it may be for you - you cannot allow the two of them to be alone unsupervised. That might mean one of the kids needs to stay over at a friends house until you come home or you hiring a college student or a local Grandma to hang out with them in the afternoons after school.
But what if it has been going on for awhile? Then the first thing you need to determine is is it mutual. Do they both want to be doing it or is one of the kids pressuring the other. ( And the girl could be pressuring the boy as easily as the boy pressuring the girl.) If one of them is pressuring the other- then you have to get the one who is pressuring the other out of the house asap. Stay with their Dad, their grandparents, an aunt - anyone. You cannot have them continue to live together with you until they both receive counselling and help.
Then contact AASECT.
http://www.aasect.org/ Find a professional who can help your children.
Moms are the best teachers to teach their son's about sexual behavior. Moms have been the one's teaching their sons about sex since time immemorial. So don't worry- just do it.
Please write and let me know how things go. If you poreferto write privately - email me at susan@sexsupport.org.
I will be thinking about you and wishing you and your children the best.
Susan