Okay so my girlfriend and I have been dating for a little over a year and a couple days ago she asked me if she could marry someone else. Naturally  I needed more context because though we have our issues (see my r/relationships post for more info) we are in a good place right now. 

Now for the context. Her friend's relative is in danger of being deported to Venezuela and pretty much his only option to stay in the United States is for him to get married. So her friend asked her if she would be okay with him marrying her long enough to obtain citizenship. However she does not want to take a break from our relationship to do so, she is willing to continue to date me while being married to him.

When I told her that I don't want to become a mistress in a relationship that I have been in for almost a year, she claimed that though we would still be dating, she would just be married to him and learning enough about him to prove the marriage is legitimate and keep him here. She says that everything would be fine between the two of us but I am still not comfortable with the idea. 

I should also mention that I have not met the friend or her relative so I cannot say if he is a good person. And my GF has been friends with her since high school (we're sophomores in college). She has not met the relative in danger of being deported.

So AITA? I don't think I am but the more she brings it up, maybe I'm just being selfish and should just put my feelings aside.