Is it normal for 6 year olds to do this??

I have a 30 year old sister who has a 6 year old daughter whom I am extremely concerned about. A couple of weeks ago my sister asked me to babysit her daughter for three hours or so. I agreed since I felt like I needed to bond with my niece more. When I arrived everything was normal and my niece was your happy little girl who believed in fairies and dreamed of marrying a prince charming. My sister left and I told my niece I was making chips for lunch. When I served it to her, she took one chip and sucked it in a way you would describe as giving a man a blow job. I told her to chew and she dutifully did so. I thought I was imagining things and it was one of those days where I kept thinking dirty. Later when I was watching tv, my niece sat down next to me and asked me a question where I realized I was definitely not imagining things. "Do you have a dick aunty?"
At that point I felt so uncomfortable that I was just about ready to leave the room but I kept my cool and laughed.
"No why would you ask? And that is a very rude word and I would like it if you never say it again."
My niece explained to me that "Alfie" had a penis and she wondered if some women had them like men too. I was so shocked at her innocent comments, as if she were asking me things that were totally normal for her age to ask. Furthermore she told me that she had lots of "fun" with her teddies.I asked her if she had learnt all this from anyone at school or home and she shook her head though she seemed hesitant. Later i told my sister all of this and she said she would look further in to this (my niece's dad is hardly around so he couldn't have fed her with this inappropriate information). A few days later my sister phoned saying she caught my niece humping her toys furiously. Just two days after that incident my sister's husband found a list of porn sites on the house computer's internet bookmarks. Is it possible for kids this young to know how to use the internet or be so horny? I've mentioned this on Yahoo but people ridiculed the story. I hope I get more helpful feedback here.