On the Facebook event page, Sister wrote:
As far as food goes, we can set out a list of "essentials" and see what everyone would be willing to bring. Of course, you're always welcome to bring whatever you'd like too!
The List for essential food (for now):
Burger patties
Hot Dogs
Buns
Condiments (ketchup, mustard, mayo, relish)
Cheese, lettuce, tomatoes
Drinks/Cooler
Chips/Dip
Sides are whatever you want! Deserts, salads, dips, fruit, etc. What's your "go-to" dish?! Bring it =)
Here's the problem . . . she never asked for people to post what they were bringing. For all she knows, everybody is bringing something.
When no one said what they were going to bring with a week left until the get-together, Sister wrote:
Alrighty folks, let's start getting the food together! Who's wanting/willing to bring what?! BIL and I will provide a grill.
From that, I think the idea is to bring enough of "whatever" for everyone and not say enough meat to grill for a couple/family unit on the provided grill.
Okay -- that makes a little more sense. Your Sister
did make a list and
was organized (to a point.) It seems that she's excited for this get together and was hoping everybody would jump in with as much enthusiasm.
People are just not responding.
Hopefully, her last post "
Alrighty folks, let's start getting the food together! . . ." will wake people up for a response.
Is Sister asking you for advice? Or are you trying to be helpful with a sisterly nudge to make her party a success?
How do I tactfully bring up that she should amend the invitation?
At this point, after reading the initial "invite" I don't think her page needs to be amended with the "bring your own meat" thought. (Changing my mind, here. Being wishy-washy, sorry. Could probably change my mind again.

)
Sis already has offers for enough meats, so she's good to go there. She should personally call the people who offered the meat to let them know that there will only be 20 instead of 40 so that they don't over-buy.
Is there a polite way to say something on her page like "If you don't bring a dish, you don't get to eat?"
As for the amendment . . . suggest that she posts what food items will be available.
"We have burgers, hot dogs, drinks, fruit tray, chips so far . . .
If you're planning to stay for food, please post here as to what you're going to bring.
If you're not staying for food, please let us know."
This is a toughie. Your sister is in a bind due to wording in her initial invitation.
I say that if she still doesn't get any responses, she should be prepared to bring paper goods and condiments. Other than that, she should go with the flow and let whatever happens happen.
It's a good learning experience.
eta: I've never organized a pot luck, so my thoughts may be completely off base. However, I do believe that organizing a pot luck and hosting a meal are two completely different things . . . I, too, am learning from this thread.