If she can't afford to host a proper shower, and I don't mean elaborate or crazy - just cake and punch at a park is fine - then decline it. People should not be expected to bring food with a gift.
That being said, I think it's a bit much for you to expect your broke sister to host all of those people. Maybe she wouldn't be as stressed out if you limited it to your family only. Let IL's host a shower for you and your friends can throw a separate shower - or maybe they won't, but it's not your sister's responsibility to include everyone. If IL's and friends don't host showers for you, then too bad. I don't think it's your sister's responsibility to host everyone in your life.
And I have no words about a pot luck wedding and a pot luck shower. It's beyond tacky.
FWIW my sister isn't the only one hosting the shower. It's her and 3 of my SILs who are not in the same financial situation as her. Honestly, when I accepted I figured my "better off" siblings would cover most of the financial stuff and it would just be a group effort of planning, not paying, since that's how it is during holidays parties.
I left a message on my SIL's machine that I want her to call me whenever she gets a chance. Last night she asked me to get in touch with her with what I want in terms of theme, time, date, guest list etc. so I'm wondering if they just aren't all on the same page... or I'm missing something. Since my sister said she's not doing a theme or decorations, yet my SIL specifically asked about them? This might all just be a misunderstanding and my sister means a pot luck among the siblings, rather than a full blown "potluck shower." Idk, either way I figure the best thing to do is to ask another host if I'm just reading the situation wrong.
As far as my wedding goes, I'm surprised that two sentences on the subject brought up so many responses. It was 2 years ago and it was very informal. I just called people, or did a facebook invite and said I'm getting married at a local park. If people want to come they are more than welcome to. It organically became a potluck since people decided to bring food on their own.