Oh, this is so sad. I lost a kitty like that once, years ago. She gave birth to two kittens, and still looked pregnant. I had not educated myself at all, and did not even call the vet. By the second day the cat had died, and soon after the two kittens she had birthed were gone, too. This was a barn cat, and I had spayed her sisters (adopted a litter of 3). But I let her have one more litter, because she was such a good mommy. I still feel very bad about her loss, and the precious babies.
If you absolutely cannot get her to a vet, call the rescues and shelters near you, and check if they will have her see the vet immediately if you give her up. Or see if you can work off part of the bill at the vets office. I believe your cat is in grave danger.
Please post and let us know. At 14, you don't make the family decisions. And your Mom sounds like she is overwhelmed. But this cat is your families responsibility, and you are looking for help. Maybe you can even ask for aunts or grandparents to pay for her vet visit as your Christmas present? Best of luck with your kitty. I hope my fears are wrong.