Is my crush a slut?

She's dating my stepbrother. On the face of it, she seems like a really nice, sweet, innocent, smart, outgoing girl. However, in August 2011 in a 5 day period I found out that she gave 2 guys bjs she wasn't even in a relationship with. And in November 2011, a week after my stepbrother broke up with his girlfriend, her and my stepbrother were drunk at a party and she took him into a port a potty and she gave him a bj. She the gave him bjs regularly for 3 weeks until finally my stepbrother was like "we're dating right?".

Anyway, they've been dating a year since that. Me and my stepbrother both have rooms in the basement of our house. I was watching tv in the basement one day and the two of them walked to the bathroom. She was wearing only a t-shirt that rose to her upper thigh. I don't know if she was wearing underwear or not. And another time in summer she was wearing yoga pants and one could see that she was wearing a thong if you could observe.

Another time my Dad opened their bedroom door without knocking and they were naked and she was giving him a bj.

This other time I went upstairs to get a drink and she was getting a drink from the fridge wearing nothing but her winter coat.

Another time I was waiting fo the two of them to get out of the bathroom and when the door opened she was wearing nothing but her bra and panties.