In matters that pertain to both husband and wife, or to the children, a "mixed" couple follows the husband's tradition, whether Sephardi or Ashkenazi. In matters that pertain to the wife alone, she is permitted to follow her family's custom. When it comes to Taharat HaMishpachah, for example, the husband's custom is followed. Mikveh immersion itself, however, is generally based on the wife's mother's practice. If there are specific concerns regarding a particular practice, an individual question can be asked.