If there was a job posting for a stepmother it might read:
Woman needed to raise someone's else's kids who will fight you and test you and might even hate you forever. You may get hit with unearned nicknames like Evil Stepmother or The Homewrecker. Job requires long hours over many years for no pay and little gratitude. Expect scant support from their father and possible enmity from their natural mother.
Who wants to apply for that job?