She can claim any one of you as a qualifying person for head of Household if she pays  more then half the cost of maintaining the household itself for the full year, where a qualifying person lives for more than half the year.    That means she pays for the household, which would include the mortgage, property tax, utility bills, maintenance costs etc.,, She does not have to pay the costs of supporting the qualifying person.  If your rent payment exceeds half the costs of maintaining the house, she could not be head of household.