I sympathize with your dilemma - it is hard to participate in something that is more than what you want. It sounds like you want them to save money, which is admirable. However, it is very difficult to tell people what they are allowed to give you. I think we get a little over-zealous these days with endless gift registries - I recently received a wedding invitation with a website address and a long list of "acceptable" gifts, plus instructions about wearing black tie. My husband and I were so offended, we sent nothing. (We didn't know the couple, only vaguely knew their parents.)
You should also keep in mind that you are experiencing all kinds of new emotions, due in part to hormones and body changes, and due in part to normal anxiety about what the future holds. It also sounds a little like you don't feel you are worthy of the attention. You are trying to cook and buy food, but this is a gift THEY want to give YOU. It's okay to make sure there is some food that you can eat (either because of general dietary requirements or because you are sensitive to certain things right now), but you really can't tell them what to serve others. It shouldn't matter what all they serve - if you go to a party at someone's house, do you care what they serve or what they spend? Can you find a way to just enjoy it?
If, however, the shower is too large and you are uncomfortable with the size of the guest list, you should have some say there. It is nice that people want to celebrate your new phase in life, and that they want to share your excitement. That's what the guests are doing, and I think that's what your family is trying to do, although they seem to be using steamroller tactics that are making you feel overwhelmed.
Maybe you can just let them do their thing - don't listen to all the planning, just stay out of it. Certainly don't cook or shop - it's important to learn how to accept help, as you will be needing it when the baby comes. You will have to be the kind of mom you think you should be, and when the baby comes and as she grows, YOU will be the one who will have to decide on her foods, discipline, values, etc. Save your strength for those days if the others try to "help" so much that it is interference. But maybe you can give them the gift of allowing them to fuss over you and help you celebrate and build up your supply of baby items. You might enjoy ooohing and aaahing over cute gifts and benefiting from the thoughtfulness of other, experienced moms. You ARE worth it and deserve to be treated to a nice celebration. Let them help feed your excitement rather than see it as a battle. Do you mother and sister know that you are not planning to have any more kids, at least for a long time? That may be why they have put so much emphasis on this, figuring it is their "only chance" - or they may feel that you are NOT excited and are trying to make you excited by going overboard.
I've been to weddings and showers that are NOT how I would have done it, but I can enjoy them because they are someone else's party. I don't wear the same clothes as a good friend of mine, but I can enjoy her taste FOR HER. If someone throws a party for you, you can enjoy being the recipient of good will and love of friends. Hopefully your stress will reduce as your pregnancy progresses - you need to take care of yourself and be good to yourself.
Good luck!