Pay for the dinner, casually suggest that you really enjoy having BBQ's with the family and that you should have one some time in the future. Depending on the sister & boyfriends living situation they might be able to host. Suggest you could all bring some food.
That way you suggest that you like hanging out with the family and would like to do something together in the future, hosting a BBQ will leave the sister & boyfriend feeling they have something to offer in return for the paid bill, and having everyone bring a dish reduces the costs on their side. So they don't end up being put off by the thought of spending cash.
If it bothers Getbigger "X"'s girlfriend, suggest she take him out some time or buy him something (or put out for a change).
Alternatively, fake food poisoning and demand to speak with the chef. Spit in his face and exclaim that the bloody steak you ordered was well-done and that you will not accept this. Write a review on your highly decorated 'Yelp' account and threaten to sue the guy and the whole branch if you do not get the food for free, preferably in a doggybag to go.