The Head of Household rules for when you are married but not living with your spouse specify that the dependent you claim on your tax return that qualifies you for HOH must be your child. Your sister does not qualify you for Head of Household
See IRS Publication 501 Considered Unmarried on page 8 - https://www.irs.gov/pub/irs-pdf/p501.pdf#page=8
"Your home was the main home of your child, stepchild, or foster child for more than half the year"
