I really don't like my brother's girlfriend, how to I talk to him about it? (I don't want revenge or anything immature like that)?

First off, my brother and I are really close. he is younger than me (he's 18, i'm 21), and pretty naive. I'm used to sticking up for him because he's not good at realizing when people are making fun of him until it's too late. We were both working in this same place and there was this girl there. She was pretty rude and dismissive towards me, but apparently she and my brother ht it off and started dating. I would have thought she'd get nicer to me, but she actually got less and less friendly, and refused to even make eye contact with me. I could overlook that, if it wasn't for the fact that I think she's manipulating my brother. At first I thought that I was just mad that he was dating someone who was so rude to me, but there have been a couple times when I think she's just using him. For example, she made him buy her sister a really expensive present for the christmas grab one year. I want to talk to him and tell him enough is enough, but I have told him that I can't stand his girlfriend in the past and it always leads to a fight, which I hate because we're usually so close with each other. For the record, neither of my parents like her either, and my mom seems to think she's not a nice person either. I'm pretty sure I want to try to have a non-confrontational conversation about it, but how do I approach it? Do I tell him I think she's using him, or just say that I don't like how she treats me and let him figure things out on his own?