The rules are different when you are Not married. When you are Single you can claim as a dependent someone who is a qualifying person to make you eligible for HOH. Your sister would be a qualifying person.
See IRS Publication 501 for Head of Household page 8 - <a rel="nofollow" target="_blank" href="
https://www.irs.gov/pub/irs-pdf/p501.pdf#page=8">https://www.irs.gov/pub/irs-pdf/p501.pdf#page=8</a>
