Is it possible to report marriage fraud?

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I have a family member who is planning on marrying someone as a favor to obtain their greencard. She is in no way in any relationship with this person,thinks that she can live separately from his and see toher men. I think she is making a really terrible decision, but she is family and I don't want to see her get in any kind of trouble. She has gone around telling people exactly what she is doing and I am worried that someone who doesn't approve might report her. so i wanted to know if it is possible to report marriage fraud and what would her and the guy's punishments be if caught?