A direct abortion, in which the intent is to kill the child either as an end or as a means to some other end, is never permissible for any reason whatsoever, even to save the life of the mother. It is perhaps possible that you have not understood the situation correctly or reported it accurately, so I urge your niece and her husband to contact the
National Catholic Bioethics Center (NCBC) for help in assessing their case. We must pray that the NCBC can help your niece's doctors in finding a treatment that will seek to save both mother and child. But if, God forbid, that is not possible, we must both pray for a miracle (
St. Gianna Beretta Molla would be a fine choice to petition for intercession) and for your niece to have the grace and strength to be willing to lay down her life for her child (
John 15:13).