Sounds like the opposing candidate next election needs to take this and run with it as there are only two reasons she would do it:

1. She deliberately broke the law by marrying someone solely for them to gain access to the US.

2. She an incestuous pervert.

Either one could be stated and which she actually is could be left to the court of public opinion.

It's not slander if it's true. One need merely give both alternatives and at least one of those statements is true.