My brother's ex girlfriend is upset with me because I knew he was cheating, is she right?

I knew he was cheating for nearly a year but didn't want to get involve too much. I tried talking to my brother about let her go but he continued cheating. I couldn't snitch on him, just couldn't. He admitted himself that he would cover up for me too if I was the guilty party.

She found out elsewhere, unfriended me on FB, considers me as the worst friend ever. She was very upset with me. Is she right to be upset with me? Her anger should just be focused on my brother, he's the one that cheated. As his older sister, this was a tough position for me.

My brother's ex girlfriend is upset with me because I knew he was cheating, is she right?
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