The problem isn't the daughter, y'know. The problem is your gf and she didn't do an incredible job of raising her kids if she allows one of them to rule the roost at age 10. Have you ever tried to discuss your discomfort around her daughter with her? Why does she let a child walk all over her? Particularly when the other bio-parent isn't in the picture.

I'm afraid Winterglow is right - your girlfriend has seriously fallen down on part of her job of parenting if she has allowed her daughter to grow into a vile little overbearing dictator, whom others don't want to be near.   In your position I would suggest that your girlfriend attends some parenting classes, or maybe some Zoom lessons in parenting, or at least reads some books or watches some episodes of Supernanny or similar.   For yourself, I would strongly urge you not to move in with her unless this changes, as I can't imagine you will be able to put up with this for very long.  Keep your own place, but don't keep silent about the changes you would need to see in the status quo, and don't accept any rudeness towards yourself from the 10 yr old.  You are absolutely right in not wanting to expose your own children to this monster your girlfriend has singlehandedly created. 

I'm with the others - it sounds like your GF did the opposite of an incredible job raising her kids. OK, maybe she did it alone, but lots of single moms do a much better job than this. Her poor parenting is the reason this girl behaves this way. You already know that, because you've said you wouldn't allow your kids to behave that way.

Yes, you need to be honest with her and see if she's willing to make changes.

Being honest with your GF has three possible conclusions: she changes her parenting for the better, which helps your relationship; she does nothing, which continues to damage your relationship; or one of you ends the relationship based on the outcome of that honesty. But, you have a 1/3rd shot of her making a change.

If you aren't honest, while it's possible that she'll spontaneously see things going poorly and change her parenting, that is unlikely to happen. The predictable outcomes are things stay the same and you stay, or things stay the same and you leave. Either way, not talking honestly about the situation offers almost zero hope that things get better and likely only prolongs the inevitable, which is the end of the relationship (since it doesn't appear that you're interested in moving beyond where you're at in this relationship).

When you have the ability to be honest, be honest. You don't have to be brutal. Even if the relationship ends, be honest as to why. It may not click with you, but hopefully it will click at some point that she needs to parent her daughter better if she wants an adult relationship.

The two biggest mistakes I made in my relationship where going in vacation with SOs unruly children, I still have PTSD from the experience, an letting them move in. Right now we are one argument away from me kicking them out. If you really like this girl keep yours and hers separate and do not try to blend your families.

I'm currently married to a person who is permissive and allows his kids to treat him badly (and occasionally me).  I'm not going to sugar coat this it's awful and I'm so tired of living with it I'm considering an exit stategy.  Here is what I would look for in the conversation.  Does you GF understand and recognize the bad behavior or does she downplay it and say it's not bad?  I think it's a very important distinction becasue if she doesn't see this as bad behavior it can never change.  So run for the hills and don't subject your children to hers.  If she does recognize it and additionally tells you she's parenting out of guilt (probably becasue her childs father was not involved) I think you have hope.  However; I would really look for change and additionally I would ask if you are expected to deal with the bad behavior or if you are empowered to correct it if it is directed at you or your children.  Those are important aspects of this problem.  This is definatley a red flag and I would want to see change before I agreed to a vacation or any other contact with your kids.  Good luck to you I hope it works out!

One of the hardest jobs as a parent is to shape their kid into someone likeable. Without this skill, the child will have a very difficult life, especially in adulthood. 

Time to cut the girlfriend loose.

It won't get better and you said yourself that you  don't want to expose your kids to that.

Your GF didn't do a great job of raising her daughter. If she did she wouldn't act this way.

My sister's kids thought they were so funny by making fun of their mother, calling her stupid, talking back, throwing fits if they didn't get their way.  It flabbergasted me that my sister allowed it. My own daughter, after visiting with her cousins, came home and started down that same path with me. Once. I was a single mom by then, and my sister was married, and her husband (the kids' father) never intervened. I don't get it. Children of DIvorce do not get a pass on basic civility and respect, and children from intact families can be jerkwads too.

Two of my sisters kids turned into lovely people, but only after they were out on their own. They must have learned basic manners from others. One remained a total entitled jerk.

Now project this situation with your GF's daughter into the future. What will this child be as a teenager? Young adult? If you can't stand the mouthiness of a child, wait until she's a mouthy teenager with an even more colorful vocabulary. You've got to address it with your GF, and the best and most loving thing you can do is be honest with her. She needs to improve her parenting skills, and I hope she appreciates that you support her in that journey.

And if she won't face it and won't correct it, then you have to do what is healthy for you and your kids.