I think the OP may be feeling more frustrated because it's coming up to Christmas and many people find their finances stretched thin to breaking point at such a time - and not by having huge celebrations but just by trying to have things be a little bit nicer at this time of year.
OP, I hope you can find a way to attend that doesn't break the bank.
I agree with this.
OP, I'm sorry for what you and your family are going through.
Several previous posters have given you great suggestions; I have one myself. I think you and your mom should just bring one dish. If anyone asks why Mom didn't bring a meat dish, say "It just wasn't possible."
Check your local grocery stores to see where you can get the best deal on potatoes. My local Aldi has a 10-lb bag of potatoes for $1. Get a head of garlic (usually about 15 cents). Cut off the top of the head, so that most of the cloves are exposed. Drizzle with a little oil, wrap in tinfoil, and roast at 400 degrees for 30-40 minutes, until garlic is soft and smooshy. Mash the garlic together with the cooked potatoes. Butter and/or milk are a nice addition, but if you don't have any, no worries, just add some salt and pepper. The garlic makes it taste very rich and flavorful. This makes a TON of potatoes and should cost you $2 or so.