You do not withhold or pay social security and Medicare taxes from wages you pay to:
- Your spouse,
- Your child who is under age 21,
- Your parent, unless an exception is met, or
- An employee who is under age 18 at any time during the year, unless performing household work is the employee's principal occupation. If the employee is a student, providing household work is not considered to be his or her principal occupation.
Since sister is not listed as an exception, you must withhold Soc Sec and Medicare.
https://www.irs.gov/taxtopics/tc756.html
