Head of Household in Taxes (NOT SINGLE)

I am "Head of Household" I support my sister in law who is working on getting her disability and does not work has -0- income why cannot i pick head of household this is wrong. I pay all her medicines, medical expenses, transport to dr. etc.   Not being able to put head of  household which is what I am is putting me at a higher  tax bracket.   She does not help me financially with anything. I even have to help her with my nieces' issues up North (her daughter). Iam not single in this case since she is family yet your software forces that option which reduces the taxes I am eligible for.