Issues with "best friend"

The girl I am with has a friend who she considers her "best friend" because they grew up together. They don't seem to really like each other though and show it a lot. I am friends with them both, but when my girl talks shit about her, I always go to the defense of the friend. Reason being that I am loyal to my friends, people talk crap about them and I don't like it. When I do that, my girl gets upset and says I don't even know and that I am basically not allowed to have an opinion on the whole thing because they grew up together. I feel like I am only doing what is right and she shouldn't be putting me in that situation. I tried to tell her that, but she made it evident that any further talk about the subject was upsetting her. I know it is going to come up again though because these two are practically sisters. I don't think it's fair that I should be forced to take sides.

So how do I dodge this situation or make her understand that I am a good friend and will not listen to anyone talk shit about my friends without speaking up.

Try to change the subject, and if that doesn't work just calmliy tell her that these converstions make you feel very uncomfortable and would like her to refrain from this talk. And if that doesn't work, let her know that you are going to walk away from her til she gets the hint that it will no longer be tolerated. Suggest to her if she has this many issues with her, she should be talking to her about them herself because b itching to you does nothing for the situation.

This may ruffle her feathers a bit but it will earn you respect.

Is it her friend or your friend your gf is talking bad about? If it is her friend, then just let your gf vent and don't worry about it. That is between them. But if it is your friend, then defend your friend regardless of what your gf thinks about it. The friend will probably outlast the gf...

Good luck.