The mother, Lulahel, is actually a being close to a half elf, a heibi. She did not sleep with the Emperor but got pregnant of the twins by using the Emperor's blood (not specified how) and her origins as a heibi. She wanted the Empress seat but instead, was forced to the Emperor's decision, which is why she made a contract : he cannot approach the twins while she lives. She blames the twins for being the reason why she herself is stuck.