I've just found out and i'm in shock.
By this point we had been seeing each other for around 3 months and he had been finished with his GF two months before we started seeing each other. This was the wedding of his cousin as he doesn't even see her on a yearly bases.
Maybe she did get an invite before the wedding but surely she shouldn't of attended?
I've only found out about this 8 months after the wedding as I was late night FB stalking someone completely irrelevant. From the FB photos she's in the family photos + sitting next to him at dinner.