It doesn't sound to me as though the tabby one is feral. She may have been dumped, and linked up with the black one. I really do hope however that you trap them both (especially the pregnant one) and get them spayed. Babies being born this time of year are likely to freeze to death, or become sickly from the bitter cold. It's way too stressful for cats to give birth in this weather, and may be the end for mom to be too. Are there any low cost or free TNR clinics that you know of?