I like the idea of people bringing casseroles and dishes that can be frozen for the time postpartum, so that you will have access to a supply of nutritious easy meals, even on those days where everything seems to conspire against you.
How about suggesting to your sister that instead of a baby shower she organize a mother blessing event. That would be an event where things focus on you and your upcoming birth, emotional support you can draw on when the going gets tough, during labor and postpartum.
The focus of such an event is not as commercial (even though some guests may still ask if they can bring a gift and then you can respond accordingly) so it may work better for you.
If you and your sister are not familiar with the a mother blessing, there is lots of information on the internet, and there are books with many ideas on how to plan one.
Here are a couple of suggestions:
Mother Rising - The Blessingway Journey into Motherhood by Cortlund, Lucke and Watelet
Blessingways - A Guide to Mother-Centered Baby Showers by Shari Maser
Such a mother blessing would pamper YOU a little bit, which you deserve. The pampering, emotional and spiritual support of yourself will then in turn help you be a better mother to both your children, as you will receive suggestions and positive stories and experiences you will be bale to draw on.
First take care of yourself, so that you can take care of your family. Let your family and friends celebrate you and your pregnancy, and let that be the focus, and not the gifts, which you're uncomfortable with anyway, it sounds like.
Best wishes and and enjoy the social get-together whatever focus it will have! :-)