Can I get my father's marriage annulled?

My father had very advanced prostate cancer, stage 4, it was all over his body, and his mental state was getting worse and worse as time went by. There were times when he didn't know what was going on. Unfortunately, he passed away like 3 weeks ago or so, on 12/30/2014. His girlfriend went behind everyone's back and supposedly "married" him. It got to the point where she was doing all the talking for him. We'd ask him how he felt and he would just stare into space and she would answer our questions. She got a marriage license on the 26th, but I don't know when they supposedly got married. On the 27th, he had completely lost his mind, then on the 28th, he was hospitalized and then passed away on the 30th. All I know is that when my father was ok, a while back, he didn't want to marry her. I don't know what her intentions are, but I don't want them to be married legally. I even think the marriage certificate could be fake. So what can I do to get the marriage annulled?