The U.S. Department of Labor has "clarified" the reach of the FMLA by offering an interpretation of the meaning of "son or daughter" under the FMLA.  Under the FMLA regulations, a "son or daughter" is defined as:

a biological, adopted, or foster child, a stephchild, a legal ward, or a child of a person standing in loco parentis, who is either under age 18, or age 18 or older and ‘incapable of self care because of a mental or physical disability’ at the time that FMLA leave is to commence.

The new Administrator’s interpretation (and first issued under the FMLA) provides some examples where the the Department would find a parental relationship despite the absence of of a biological or legal relationship such as:

• where an employee provides day-to-day care for his or her unmarried partner’s child (with whom there is no legal or biological relationship) but does not financially support the child;
• where an employee who will share equally in the raising of a child with the child’s biological parent would be entitled to leave for the child’s birth ;
• where an employee who will share equally in the raising of an adopted child with a same sex partner, but who does not have a legal relationship with the child, would be entitled to leave to bond with the child following placement, or to care for the child if the child had a serious health condition, because the employee stands in loco parentis to the child.

You can access a complete copy of the Administrator’s interpretation here.