Is my fiance's sister normal?

I'm 25 and my fiance' is 26. We've been together for 8 years, since I was 17. We've lived in our own apartment together for the past 3 years. His sister is 16, she was 8 when we started dating. I always noticed that she was very 'babied' by their mom, but thought it was ok since she was only 8 at the time. She was also very clingy with my fiance, but I also thought this was fine as she was only little and I was also like that with my big brother at that age. As she got older things started getting weird. They've always played these wrestling games where he picks her up and body slams her on the bed, but she started making them become kind of sexual. When she was 14 she asked him to bench press her like her used to when she was little, so he said "No Kelly you have boobs now, I can't do that" then she said I don't mind and grabbed his hand and put it on her boob! I thought that was super weird. Another time they were wrestling in the room and he pulled her pants off by accident, he said I don't wanna see that! and turned away, but she stood up really slowly showing him everything on purpose. She would always want to sit with us when I came over to visit. Whenever she and I were alone she would tell me about all the boys at school she wants to get with and what she's going to do to them when she does. But as soon as her mom or dad or brother are around she acts like a baby again. She's not allowed outside in the pool or yard unless someone is watching her. She's not allowed to stay home alone even for a few minutes. She has to still hold her mom's hands while she crosses the street. I asked her mom why she does this and she said it's because Kelly is still not "street-wise" and doesn't think of safety. She asks her brother pick her up and always wants someone to carry her or piggy back her like she's a little kid. When she lays on the couch watching tv she drinks coke out of a baby bottle! Then she gets all clingy and touchy when her brother is around. A few weeks ago when we were visiting she sat on his lap with her arms around him, rubbing his face and ear. She looked at me and said in this sexual voice..mmm are you jealous? Sometimes when we visit his mom, him and his sister will go into their mom's room and lock the door. I always thought they were just wrestling in there but the one time I walked in when the door wasn't locked and she was sitting on his chest up by his neck with her whole vagina basically in his face. I had no idea what they were doing but they said they were just playing. He got really mad if I came into the room after that, saying he never gets to spend time with his sister when I'm around. It's just really weird..a few times I've walked into lounge and they're holding hands while they watch tv or she's playing with his ears or hair and lying on him, like they're a couple. My fiance also acts really childish with his mom, like he whines if she doesn't buy sweets when she knows we're coming over and he nags her like a kid. When he's with me he's totally normal, he just acts weird when he's around his sister. I don't think this is normal..she's almost 17! And it just doesn't seem to be ending. She's super sensitive too, but I think she's just putting on a good act to get attention, even though she gets plenty of it from her family! She always chooses the worst times to want to show people things then gets really upset when they don't want to see. Like once we stopped at my fiances moms just to drop something off after work, we were both really tired after a long day and weren't planning on staying, but Kelly comes all excited and says "Come look at the photos my friends and I took at school today!". I didn't feel like looking because I know how she explains each photo in detail and it takes forever but I said ok and asked how many photos are there. She said over 300...so I asked if we could rather look at them next time we come over because we're exhausted from work and wanted to get home. So she started crying! My fiance was like "awww it's ok Kelly, I'll look at the photos if Kaja wants to be mean." A couple weeks ago when we were there watching the soccer world cup final, she wanted to show me a video on youtube in the middle of the match. I told her wait till the match is over, then I'll watch it. So she went to sulk in a corner and didn't want to talk to me for the rest of the night. It's so childish for her age! Every time we're over to visit she wants to sit on his lap, and has even tried sitting on mine a few times. She pokes us and jumps around like a 5 year old, but then acts all sexual and talks all sexual when she gets the chance. It's just so weird watching this tall almost 17 year old with big boobs jumping around like a kid and asking to be picked up. Any advice? Confronting my fiance won't work, he gets really upset if I bring her up