My (41M) girlfriend (28F) can’t get along with my 5 year old daughter. Is there a way for me to fix it, or should she (girlfriend) move out?

My girlfriend and her two sons (5 and 2) moved in with my daughter and me a year ago. At first, my girlfriend and daughter got along, but over time, they're relationship has been strained. These days, my girlfriend just avoids my daughter when they are both home unless she can hear my daughter giving me attitude from the other room. Then, she'll chime in to criticize the brat that my daughter is being. My daughter is being a brat in those moments, but I don't cave to her will, nor feel like I need any parenting assistance at the time. I would like for my daughter to get more from my girlfriend than just a mean future step-mother who has nothing to do with her except to criticize her. I expressed my concern to my girlfriend, and she's now offering to move out.

What do you folks think I should do?

Edit 1: I did not know that the word brat could be so offensive to some.

Edit 2: Some of you seem to think you know way more about us and our situation than what I've explained, and are very critical of me. Folks, I'm paying attention and addressing the issue. My daughter comes first, without a doubt, and my girlfriend will most likely be moving out because of that. If I didn't understand how this could affect my daughter, I wouldn't be seeking relationship advice before I make a big relationship decision.