Bigamy is against the law so it sounds like your sister would have grounds for annulment. Once granted, an annulment would void the marriage as if it never took place.
If she has not done this, she would also need to file a paternity action (petition to establish parental relationship) with the court to address custody, visitation and support for the children.
Answered on Aug 21st, 2013 at 4:00 PM