Hi Megan,
I think saying “No” to his family would be very good for him. You don’t REALLY want a man who can’t stand up to even his sister, do you? Because this would be the first arranged marriage, he could get out of it if he really wanted to. If he ends up marrying the girl, it wasn’t a match anyway.
My advice is to tell him: “Contact me when the engagement is called off so we can be married. Don’t contact me if you choose her. I’m not going to hold your hand. You need to do this yourself.”
Good Luck,
Inky