When is it suitable for your girlfriend to tell your kid brother to shut up?

Sitting in my house and my 12 year old brother was niggling at my 21 year old girlfriend in a funny jokey way. He then asked her to help him find my sunglasses an hour later, perfectly suitable question... as she had just had them earlier. Only for her to turn round and say "how about you shut up?"

I was shocked and we've been struggling to come to terms with the situation. She says my brother was cheeky and that he deserved everything she said to him. I disagree and would never turn round to her sister in HER house and tell her to shut up, so what gives her the right to say it to mine.

If she had a problem with my brother she should have came to me and I would have dealt with it.