Is it normal to be so hurt by my best friend and cousin?

My cousin from Germany is living with us for a few weeks to learn English. She's a year younger than me and we get along really well. I like her a lot.

My best friend (who also happens to be my fwb) and I are extremely close. He's an amazing friend, he always makes sure I'm safe, he's a great listener, he never breaks a promise, and has been there for me and has talked me out of very bad situations. Yet, ever since I've known him, he's the type of guy who goes after easy girls, struggles with addiction (he got into it pretty early on because of a traumatic event... it sucks but he's getting better) and is just a general asshole when it comes to relationships, which is what makes what we have really special.

Anyway, he was over one night, and I had just introduced him to my cousin and was showing him some things that I was working on when I realized that he wasn't in the room anymore. I went looking for him and found him laying on my cousins bed, talking with her. The three of us talked for a long time. When he left, he texted me that he really liked her (which is super unusual). I told him before my cousin even came that he shouldn't go after her, because of his history of treating girls like shit, and he promised he wouldn't even look at her. I brushed it off, but he won't stop talking about her. Whenever he does, I get this awful feeling in my stomach and I feel sick and angry but I don't know what it is. I didn't think i had feelings for him, but now with this whole thing, I wonder. It doesn't make any sense to me. Last night he drove my cousin and I to a party (it was her first American party) and she and I both had a bit too much to drink. He kept on flirting with her, and so I pulled him aside and asked him what was going on, because he said he wouldn't. Again, he said that he would stop, but he didn't. He kept it up but instead left a hand on my back, kinda rubbing and pulling me in until I was leaning against him.

I don't understand why I'm so upset about this, it doesn't make any sense. It hurts that he keeps on saying that he'll listen to me, and it's confusing why I'm so stuck on them not being together. I've seen him with so many other girls and it's never bothered me like it does now.

Is this normal? Why am I feeling this way?