Alexa, play "All You Need Is Hate" by The Delgados

Maybe they are first cousins?

Or half-sisters???

So bring your sister to your gf’s (paren’s) place and introduce her👍🏼

And you haven't explained this to her parents because???

And he owes them an explanation, why?

He doesn’t OWE them anything but it’s not a smart move to not give a shit about how your partner’s parents feel about you.

um... isn't this easily fixed? just tell the parents it was your sister. I don't get the big deal.

Does her name happen to be Cersei? Yeah Jamie... thought we wouldn't find out?

It doesn't even matter whether she was his sister or not. People in relationships are allowed to have friends, and his girlfriend's parents are being unreasonable and controlling. I'd simply tell it's none of their business.