Ok lots of good advice and comments but let me simplify this for you a bit more.

If you trust your partner let them handle their family, trust what they say about them. If you cant, it ups the chance of break up because she is already stressed.

Unless you suspect you are the other dude or in danger stay out of it no matter how curious you are. There is literally a community full of stories like this and it always back fires when people get involved too early.