When I was growing up I had nieces and nephews (I was the youngest in my birth family, so my nieces & nephews were roughly only a decade younger than I). One of my nephews (an only child) was a danger to my other nieces & nephews. We found out how dangerous during a family gathering when my nephew (at age 5, he was very big and powerful for his age) pinned down one of my little nieces and tried to strangle her. A horrible situation, but my sister & her husband handled the problem really well. ANY time my dangerous nephew was in the company of their children, his cousins, my sis and BIL kept a very close watch. Without exception, their kids were never left alone with their cousin. In that way any tragedy was avoided.

I agree with every other post on this thread - don't ever take your eyes off your daughter when her cousin is around.

btw, there is a happy ending to my story. My dangerous, mean, bully of a nephew is now a 38 year old man, a 'gentle giant' standing 6'7", intelligent and sweet-natured. There is no evidence of the demonic-acting child he was, he adores his cousins, and thankfully because an eye was always quietly kept on him after the near-strangling, nothing bad ever happened.