Is this normal that she acts this way around her nieces and nephews?

My girlfriend is 15 and I'm in college.(18). Recently my girls nephews and Neice came over( the two oldest : the nephew and Neice are not her biological though, and the boy is 12 and the girl is 13.)and there are two younger boys which are her brothers real kids. The other day we ovoo'ed and they were there. She was doing her hair and all of a sudden she started making sex noises, shortly afterwards she got up and started dancing(twerking,popping,etc) then she got on the bed and stuck her booty up in the air and did it again in front of them while they were in the room. Later that day we were leaving a restaurant and they were supposed to drop me off at home or whatever. When she got in the truck, we were sitting on the back seat and her three nephews were sitting on the same seat, and the Neice was in passenger seat. my girl started licking me on the ear and neck the she said"watch this" then she got up and licked her nephew in the ear but he just laughed and she did it a total if 3 times, then she kissed them all then asked me if I wanted a kiss(we played with each other on the way back home).the next day I ovoo'ed her she was talkin about sex around them and what we did last night, and she was making sex noises again. And then we played truth or dare while on the phone ( I dared her to get up and slow grind) for a minute straight and she did it. Then she decided they should turn off the lights and play fight. So they did that a few minutes??!!! What is this called?( btw: my girlfriend is very hot, she has a plumb shape although she is skinny, and she is not sexually active, neither am I , and she has a very nice soft ass and body, she is a freak and she often flirts with me or does things to get my attention or to turn me on or whatever( bending over to tie her shoes, wearing right well fitted clothes, sometimes loose clothes that show every detail on her rear end, such as shorts or whatever. Kiss me or sometimes let me grab and touch her) I'm curious could she be flirting with her nephews or arouse them for the fun of it in order to get my attention and make me horny or is it just how she acts and her flirtatious nature, because she freely does attractive things around them such as bending over for no reason, or dancing in front of them, talks about the things me and her do to each other, and frequently talks about sex and stuff?