It starts out like a cliché rom-com variation in which a spurned wife goes to war against the woman who replaced her. You expect hijinks to ensue and the mismatched moms in “Stepmom” (1998) to eventually unite against some type of greater evil, with hilariously heartwarming results. However, the greater evil is cancer, an uneasy truce suffices for teamwork, sadness befalls everyone, and there’s lots of speeches that sound like they were edited by a committee of Hallmark-approved psychologists. It’s a box-of-Kleenex, pint-of-ice-cream tearjerker that will make you happy it ruined your night, if you’re into these kinds of movies.