" but I'm thinking I might request her myself and tell her I think we need to have a conversation. If I can get some things straightened out, some questions answered, and be reassured she is not using my brother, I would feel a lot better. What do you ladies think? Would this conversation be a bad idea?"
I think that kind of conversation would be a bad idea. I will tell you why.
Fantasy imagined conversations almost never go the way you want them to go. The only exception to this rule is when you need to convey some kind of one-way information to a person. ("You are fired.")
You want to get some questions answered and be reassured. What if she does not answer your questions and what if she does nothing to reassure you? "Yeah, I'm using your brother. He's a big dope. So what? Got a problem with that?"
What if you feel even worse about the whole scenario than you do now? And, yes, it can get worse.
My suggestion to you, based on a lifetime of experience, is to let sleeping dogs lie. The more you stay OUT of the relationship, the more you stay neutral, the LESS they have invested in staying together. The last thing you need is for them to get the idea of "It's us against the world".
Ideally, you would like Brother to figure out for himself that his girlfriend is a big jerk. Then, when they break up, you can say "Well, Brother, win some lose some. You might remember when High School Horace dumped me. Oh... and there's this really nice new co-worker that just came into my office. She's single, and like you, she's a diehard Trekkie. She has a photo of herself, on her desk, in Vulcan ears and makeup. Etc."