I think what to say if you want to make sure they know they will be responsible for their own bill, would be to state that it will be "Dutch Treat".
However, you say that you are considering holding it at a restaurant due to the state of the economy... I would think it would be more affordable all the way around to host it at your home (or someone else's, it is all family, right?) and ask each guest to bring a dish (or salad, or appetizer, or dessert - whichever direction you plan to go with this) to share. You could be providing plates, cups napkins, utensils and drinks, for example. To me that is a lot more fun and more affordable for everyone.
I'm not sure how long before her due month you are holding this event (an dhow much room the expectant mom has in her freezer), but you could also suggest guests bring one dish to share and one dish to freeze, that can be used by the family for quick, easy and nutritious dinners after the baby is born...
Just some thoughts... Whatever you end up doing, have fun, and best wishes to your sister-in-law for her labor and birth.