I will probably, in turn, get flamed for this, but I think that if I were a child, I would rather grow up with the knowledge that my mother laid down her life for one of my siblings, rather than killing that sibling in order to be with me. I'm not judging anyone who would choose otherwise, but I would feel, as the child who kept her mother at the expense of a sibling, that our bond had been tainted. Just my $.02
As for the principle of double effect, it is important to note that the death of the baby to save the mother must not be caused by direct action upon the baby. A standard D&C, suction, D&E, or partial-birth abortion is not acceptable in these cases. Just as an ectopic pregnancy may be terminated is the child is not directly harmed (by removing the part of the tube that contains the baby), an "abortion" of a child whose mother would die without treatment that would kill him or her must involve an intact delivery via induced labor or c-section.
God bless this woman for doing the right thing.
I should have added that "double effect" terminations are not condoned by the Church, which encourages self-giving love and protection of innocent life, but that these particular cases have a different moral status than direct abortion to save the life of the mother or any form of abortion that would not save the mother's life.
However, I see the responsibility toward already born, living children as being more important than responsibility toward a child who has not been born yet. This is MO and I will stick with it. Have a nice day.:)