Victim of Bigamy

Hello I’m trying to help my sister find out some information. Here is the issue:
My sister was married to a man that she had children by after the marriage. She found out many years later after the seperation and incarceration of her spouse she finds out that her marriage was not legal. Her spouse was married before and still legally married to this woman. My question is does she have to file for divorce? She wants to get rid of the last name that she carried for so many years because she feels like it was all a lie. She knows that the children are legitimate but she doesn’t want to carry the name any longer. What does she need to do? Does she have to file divorce or annulment?

She does not have to get an annulment or divorce. Read this article for further information: