First of all, I'd be honest with your sister...let her know that you are still open to have her visit, but with your DH being let go, the food budget and extra money for entertainment will be tight.
If she still wants to come, then break out the cereal, mac-n-cheese, pasta, and hotdogs. Check out area attractions that are free.
I know that when I visit my sister, I try to be a good guest by treating sis and her family to dinner and/or go along to the grocery store with her and contribute toward her purchases. I will also bring and/or buy wine, etc.
Just be honest with her upfront. Not sure how your visits go, but if she and her family want to do some things without you, let her know that it will be okay as you are watching your pennies.
If for some reason, this is NOT okay with her, then she can change her plans and or shorten the time they spend with you.
Listen - in this economy, you are not alone. You need to do what is right for you and your family and not worry what others will think. You should, definitely, speak with her ASAP to let her know what the situation is, though, so she can adjust her expectations, and/or visit.