Obviously, this was a traumatic experience for you, but not all traumatic experineces cause PTSD. PTSD has many symptoms and it usually affects someone's whole life and impairs their ability to function normally. You have to have a number of different symptoms of hyperarosal, re-experiencing, and avoidance to be diagnosed with PTSD. A hyperarosal symptom you experienced is the constant memory of the trauma while you were around your step-sister. An avoidance symptom would be not wanting to be alone with your step-sister. I'm not going to say it's not PTSD, you should see a T to diagnose this. But I would think that you would also be afraid going around that area of town where it happened, that house, knives, someone else opening the silverware drawer, or anything brushing against your thoat if you had PTSD. If these memories are seriously and negatively affecting your life, then you should talk to a T about it.