Confused Mom:

This was not directed to you or your son, but the behavior with his sister is alarming and I would get him in to a therapist to protect your daughter as soon as you are able.

As you say you do not know what else he could've latched onto, but your computer has records of all sites visited. Check on him. Remain calm and love him through this period of confusion in his life.

Give mature direction and get him some professional help, it is a symptom not the problem