Hi, I have 2 rotts...my male Angus does not like one of my brother's for nothing in the world....they are both very friendly, but for some reason he doesn't like my brother, but i think that he can sense that my brother doesn't like dog's. Now my other girl now at rainbrow bridge..when she was a young my other brother did something to her and she never forgot and she would do what your dog is doing to your cousin. they are very smart!