A person who is your qualifying relative only because he or she lived with you all year as a member of your household is not a qualifying person for the Head of Household filing status even if he or she is your dependent.
Per IRS Publication 501 Dependents, Standard Deduction, and Filing Information, page 10:
Table 4. Who Is a Qualifying Person Qualifying You To File as Head of Household?
IF the person is your...
qualifying relative other than your father or mother (such as a grandparent, brother, or sister who meets certain tests)
he or she isn't related to you in one of the ways listed under Relatives who don't have to live with you on page 18 and is your qualifying relative only because he or she lived with you all year as a member of your household.
THEN that person is...
not a qualifying person.
For additional information please refer to IRS Publication 501.