Do not feel too bad, at least her kittens will be born in safety, and she'll be loved and well nourished enough to safely carry them. These things happen, and we deal with them as best we can. I am sure you will be sure to find the kittens good, responsible homes as well. In the meantime look around, maybe there are some low-cost spay/neuter programs around to take advantage of once her kittens are weaned.