Ann O'Neal García
Spirit on the Wall
1982
from the dustjacket:

"She raised her arm as high as it would go, gripping the rock with all the strength left in her fingers, vowing to crush her new daughter's skull, ending the misery quickly and silently. As the arm journeyed upward, it was caught by an old woman's clawlike grip..."

Mat-Maw was not going to let her crippled granddaughter die. Before the death blow could kill the squirming child, the old woman wrenched the baby from her angry mother and carried her to the dark inner depths of the cave, where no one else dared to go. The clan was afraid to follow MatMaw because the forces protected her. How else could she have lived for so many years without food? Having turned her back on the clan when they had left her mate to die at the hands of an enemy, she had chosen to live apart from the rest.

So Em, the child, had come into the world without a mother's love and with only an old woman to raise her.

This is the extraordinary story of three fiercely independent people — Em, who overcomes her handicap to develop into a remarkable artist; Mat-Maw, whose rebelliousness supports Em in her defiance of clan customs; and Em's brother, a talented cave painter who joins the two strange women so hated by the others.