As Catmandu said, YES!

Feral cats form bonds of friendship and can be intensely caring.... I have had barncats bring home strays for meals... Kibbler-fudgebutt brought home a large male cat that never tamed down enough for humans to handle but helped raise and nurture other cats of the colony.

Cats can eat a lot at a time. She may be feeding others or she may have kittens hidden. The black-one may or may not be herkitten. Friendships form for a variety of reasons (Kibbler was male and so was his friend... and they weren't brothers or related at all.) A fat belly could indicate worms or be a side affect of longterm starvation. If she is truly a feral she probably would not be having kittens now... Cats that are wild generally do not go into heat during the worst of the winter in cold climates. There are exceptions, but if she is pregnant now, she was probably a housecat once and "done wrong" by her people.

Bless you for caring for them and feeding them. The black one may eventually take her cue from the greyling and trust you. Though I could never touch Kibbler's Friend, he did purr when I spoke to him.