Honestly I don't understand how anyone doesn't like incest

I was playing with my little sister in my room all day. She was sitting in my lap almost the whole time. When we were sitting on my bed, she started playing with my hair, and I could feel her arms around me. Then she made me sit up straight and started playing with my hair again, and she was standing between my legs with her hands on my chest. I don't know why she was doing it but honestly I like it when she hugs me and this felt a lot like a hug. I don't understand how anyone can be uncomfortable with their sibling sitting on their lap. Why isn't it understood to be ok?