According to the National Archives(1) an eligible employee can to take advantage of the Family and Medical Leave Act to take care of a parent with a serious health condition. However is necessary to check the definition of parent and son or daughter. For the American law(2) parent means a biological, adoptive, step or foster father or mother, or any other individual who stood in loco parentis to the employee when the employee was a son or daughter. Son or daughter under the American law(3), means a biological, adopted, or foster child, a stepchild, 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.
Therefore even if a parent literally had nothing to do with a biological child the child now employee can leave using the FMLA since that parent became your parent-in-law before the employee was 18 years old. However an employee that has a parent-in-law after age 18 is not eligible to take advantage of the FMLA because the law don’t recognize as a parent. Under the FMLA leave it really matters if that parent is biological or not and an eligible employee must prove through documents the relationship with that parent. An employer covered by FMLA has to employ 50 or more employees for each working day during each of 20 or more calendar workweeks in the current or preceding calendar year. Then even if the employer drop to less than 50 employees they can be covered by the law if the still meet the coverage criteria for 20 workweeks of the preceding(4). Other business that does not meet the rule above is note eligible to FMLA and their employees cannot take advantage of that.
Is important to take a note that some “public agencies” and every school even the private ones are eligible for FMLA(5) without subject to the coverage threshold of 50 employees carried on the payroll each day for 20 or more weeks in a year. However they must meet every other requirement.