Final answer:
Under the "birthday rule", the health insurance of the parent whose birthday comes first in the calendar year is the primary insurance. In this case, since the mother's birthday is May 26 which is earlier than the father's birthday of May 28, her insurance would be the primary coverage.
Step-by-step explanation:
The determination of primary insurance coverage between policies held by each parent is often addressed by the "birthday rule", which is a standard practice in the health insurance industry. Under this rule, the health insurance of the parent whose birthday comes first in the calendar year becomes the primary insurance for the children. When determining primary coverage, the year of birth is not considered, only the month and the day.
In the scenario described, the father's birthday is May 28, 1989, and the mother's birthday is May 26, 1990. Since the mother's birthday comes earlier in the year, her insurance would be considered the primary coverage for the family.