This is not my situation. I was just curious as I like to bring up interesting topics...

If your girlfriend lets her Sister(a married Woman) and her friend(a recently divorced man) have sex in her home knowingly, and helps her Sister lie to her husband about her whereabouts, would you say that would necessarily mean that your girlfriend would be doing the same? Could you trust your girlfriend if you knew she was doing this?

Again, this is not my situation, but I have seen this I don't know HOW many times with different friends of mine within the last year

I would LOVE to hear some of your input