Inappropriate touching

I found out from my girlfriend that my younger brother snuck into her room and touched her butt and breast while she was sleeping. I was completely supportive and told my girlfriend that I would talk to him about it. Upon asking him what happened, he lied and said he didn't do it. Even though I believed my girlfriend, I wanted to trust my brother's responses. I don't know why, I felt the need to report back to my girlfriend and tell her that he denied it ever happening. She immediately had the feeling that I was only talking to my brother for her and I did not believe her. Later, we both decided to ask him again but this time make sure he tells the truth, which he did after prodding. He said he knows what he did was wrong and he will not do it again and he apologized. I know that my brother apologizing is not going to make my girlfriend forget the trauma she went through. After our discussion with my brother, I spoke to my girlfriend and she said she does not trust me anymore. I understand where she is coming from, but I honestly did not know how to handle this situation and clearly did a messy job of it. She says that she still loves me and that she understands why I believed my brother when he told me he didn't do it (I was in denial). But I can't help but think our relationship has been ruined because of this. It took really long to cultivate this relationship and gain her trust but this incident and the things that followed, sets it back years. I don't know how I can fix this. Please help.