Assuming that the employee otherwise qualifies for FMLA leave, the U.S. Department of Labor takes the position that the employee can get FMLA leave, with the rule for eligibility being as follows:
Quote Quoting What does in loco parentis mean under FMLA?
In loco parentis refers to a relationship in which a person puts himself or herself in the situation of a parent by assuming and discharging the obligations of a parent to a child. The in loco parentis relationship exists when an individual intends to take on the role of a parent to a child who is under 18 or 18 years of age or older and incapable of self-care because of a mental or physical disability. Although no legal or biological relationship is necessary, grandparents or other relatives, such as siblings, may stand in loco parentis to a child under the FMLA as long as the relative satisfies the in loco parentis requirements.

Under the FMLA, persons who are in loco parentis include those with day-to-day responsibilities to care for or financially support a child. Courts have indicated some factors to be considered in determining in loco parentis status include:

• the age of the child;
• the degree to which the child is dependent on the person;
• the amount of financial support, if any, provided; and
• the extent to which duties commonly associated with parenthood are exercised.

The fact that a child has a biological parent in the home, or has both a mother and a father, does not prevent an employee from standing in loco parentis to that child. The FMLA does not restrict the number of parents a child may have. The specific facts of each situation will determine whether an employee stands in loco parentis to a child.