My sons are 9 and 7, and I have a homeschool group get together once a week with kids ranging in age from newborn to 14. I think the curiosity is natural. My boys were intrigued, in the beginning. Now it's old hat for them. They don't even really notice. I started out making them leave the room, atleast until the baby was latched on. Now, I don't even do that and they don't really pay attention.

I did notice that one of the little girls in our group was extremely intrigued by my breastfeeding. She stopped playing to come over and watch. For me, personally, it was no big deal. She wasn't staring at my breast, but she was watching the process of me feeding the baby - if that makes since.

Since that first time, she hasn't paid much attention. Maybe if you don't try to get away from your stepdaughter then she might not be as interested. (some people always seem to want what they can't have).