Hi im a new cat owner, Actually I own 2 new cats, they are brother and sister that are a year old.When I adopted them, I was able to take the boy home, but not the girl because he was fixed, and she wasn't. When I brought the girl home the next day, the boy started to act funny around her. He began to start biting the back of her neck, and I mean grab on it and not let go. She didn't seem to mind too much, but it would last for 3-5 min at a time, with her finally kicking him off of her. Is this a sibling thing or is this a behavior problem that needs to be worked on. They don't seem aggressive towards each other, its just he does that to her over and over again. Its not like they are strangers to each other, they have been in the same cage together in the shelter, and they are familly.