The Wage and Hour Division issued an Administrator Interpretation clarifying the definition of “son and daughter” under the Family and Medical Leave Act (Section 101(12)) to ensure that an employee who assumes the role of caring for a child receives parental rights to family leave regardless of the legal or biological relationship.
In Loco Parentis
The FMLA entitles an employee to 12 workweeks of leave for the birth or placement of a son or daughter, to bond with a newborn or newly placed son or daughter, or to care for a son or daughter with a serious health condition. The definition of “son or daughter” under the FMLA includes not only a biological or adopted child, but also a “foster child, a stepchild, a legal ward, or a child of a person standing in “loco parentis.” In loco parentis is commonly understood to refer to “a person who has put himself in the situation of a lawful parent by assuming the obligations incident to the parental relation without going through the formalities necessary to legal adoption.”
Examples of situations in which an in loco parentis relationship may be found include where a grandparent takes in a grandchild and assumes ongoing responsibility for raising the child because the parents are incapable of providing care, or where an aunt assumes responsibility for raising a child after the death of the child’s parents. Such situations may, or may not, ultimately lead to a legal relationship with the child (adoption or legal ward), but no such relationship is required to find in loco parentis status. In contrast, an employee who cares for a child while the child’s parents are on vacation would not be considered to be in loco parentis to the child.
Conclusion
Based upon a thorough examination of the relevant factors, it is the Administrator’s interpretation that either day-to-day care or financial support may establish an in loco parentis relationship where the employee intends to assume the responsibilities of a parent with regard to a child. In all cases, whether an employee stands in loco parentis to a child will depend on the particular facts.
http://www.dol.gov/whd/opinion/adminIntrprtn/FMLA/2010/FMLAAI2010_3.htm

