The fact that you are not on the mortgage or deed does not disqualify you from being Head of Household (HoH) for tax purposes.
To be HoH, you must be claiming a dependent*. Your mother would satisfy that requirement.
Next, you must have paid for more than half the household expenses. It sounds like you have, but to be sure you have to do the math. Add up what you pay and compare that to what grandma pays.
*There is an exception for non-dependent children.
