If SCRAPS doesn't help you find a low cost clinic, just start calling local animal rescues and vets, you may find someone who will take pity on this poor kitten and treat her at a discount or for free.
If you guys are going to keep these kittens, they all need to go to the vet for shots and to get neutered at the very least. If your parents don't want to do that, you need to call a cat rescue and surrender all of them together. They are very vulnerable right now and they need someone to take them into their home, provide medical care, good food, proper socialization and protection from other animals, humans and the weather. Without that, they really don't stand a chance.
I know how heartbreaking this can be, but if you think your parents aren't going to agree to do everything these cats need, then you've got to find someone who will. If you don't, you're going to have a lot of sick cats, feral cats, pregnant cats and more kittens on your hands. The problem will only multiply.