I understand the point you are trying to make, but I don't see them as equal evils. It is not certain that bringing this baby to term will kill the mother, while it is certain that aborting the baby will kill him or her, and the baby is not at fault for being conceived during an act of rape. They are not equal evils at all.
Although, I would defer to the grandmother's judgment in this because we never have nor will live in a perfectly just world, and this decision to abort doesn't seem to have been taken without serious consideration.
It is not unlike the rationale we used to assuage the guilt we felt at the killing of innocent Iraqi children in the service of a greater good.