Under the provisions of the FMLA, an employee’s children (son or daughter), are immediate family members for purposes of FMLA. The terms son or daughter do not include individuals age 18 or over unless they are "incapable of self-care" because of mental or physical disability that limits one or more of the "major life activities" as those terms are defined in regulations issued by the Equal Employment Opportunity Commission (EEOC) under the Americans With Disabilities Act (ADA).
Therefore, if the daughter is over 18 and "incapable of self-care" because of mental or physical disability that limits one or more of the "major life activities", then the employee would not be covered under the FMLA. If the daughter is under 18 then the employee is covered under the FMLA.
Generally, in this type of situation if the doctor requests the parent care for a daughter over the age of 18, and provides a physician's statement in the regarrd, then the employee would be covered.