I think it would be fine to say something to your brother's girlfriend. Something like, "This is really awkward but you actually picked the same dress as me, just in a different color. Is there any chance you could wear something else?"
You're the mother of the bride, I think you take precedence here. If your brother's girlfriend declines to change her choice or gets offended, there probably isn't a way to politely push the issue but if she's fairly polite, it's more likely she'll say, "Oh my goodness, thanks for letting me know, I'll pick something else."
The other alternative is to buy or wear something else. The dress you got is nice but how often do we have the opportunity to be the mother of the bride? Maybe splurge a bit and get something that feels extra special to you that's still in keeping with the style your daughter is going for.
You could also ask your daughter what she thinks you should do. She might call your brother and ask if he can talk to his girlfriend, or she can help you pick out another style you like.
I think it's also fine if you and the girlfriend both wear what you were planning as they are different colors, but if you're going to feel uncomfortable or awkward on the day then it's worth taking some steps to make a change to ensure you really enjoy your daughter's wedding.