So this flirting happened 10 years ago or is happening now? I’m a bit lost. 
It appears it was going on since they met.
Like he didn't realise me and her are close. Because for a lot of people a cousin is just a cousin. So he slipped up in front of me. I assumed he was a total jackass and figured he was just the boyfriend of the month. I later learned that they were only together for a short period of time, and just shrugged.
I went away and then I came home to find out how really bad it was.
He got money and she doesn't want to take half of it, like she really doesn't want to divorce him, but she can't just have him doing endless blow and banging strippers after dropping their kids off with us to babysit them.