Actually, it says he started when his cousin was 6, and ended when Kanye was 14. Either they have a big age difference or they don't and this went on for a long time. Either way, i agree, this is not normal.

Edit: for clarity, young children "playing doctor" on occasion is entirely normal. Young children mimicing sex acts that they have seen can be more concerning, but can be innocent exploration for kids who don't know any better and ultimately be fine. But young kids continuing to do these things periodically or increasing in experimentation over years is not healthy developmental behavior. And a significantly older kid, especially one who is old enough and understands what these acts are, performing sex acts on a younger child is extremely concerning. A) it often correlates with having their own sexual abuse history by an older person, B) it correlates with an unhealthy sexual development and inclination toward pedophilia, C) it can completely shred ones mental health as they struggle to cope with the trauma as they grow to understand that they were abused, and D) it propagates those things onto the younger person in turn.