I agree. Keep her inside, confined to one room, so she doesn't have a chance not to get along with the other cats. I hear that "cat attract" will help her to use the litterbox inside. Or maybe you can try using dirt in the litterbox. These are all suggestions that I've had in the past.
Keep her inside until the kittens are weaned, and then get them all fixed at the same time. You might be able to find a low-cost spay/neuter clinic in your area.