We found out last week that things are much worse than we feared with my step-son. His older Cousin (mid-20s) has been abusing him since his dad moved out.
We found this out when we took the kids out to eat and I got a steak knife out of the silverwear rack and he jumped up from the table panic stricken saying "please don't throw that at me, I promise I will be good." He was not misbehaving at all, but he is autistic. His dad was fixing his plate, so I cuddled him and asked him who was throwing knives at him and he told us his cousin throws knives at him for running in the house. I assured him we would never throw a knife at our house, we only use them for food and tools and calmed him down.
When we got him home, he told us the cousin punches him in his stomach and groin, hits him in the back and head, and pokes him in the eyes. This is a terrified little boy of his cousin and would most likely explain the drastic change in behavior in both houses.
Please let someone know this is happening. If the cousin is doing this to him, who knows what he is doing to other children. I once had a 7 year old SN student confide in me that her 19 year old step brother was sexually abusing her. I reported it to the authorities and personally talked to the mother and father about it as well (it was the father's son.) We discovered the guy was also abusing the 4 year old sister and 3 cousins! Although the father was heartbroken, he and the rest of the family were very glad to discover what was happening. They actually filed charges on the son.