It sounds like your sister and you don’t qualify as dependents. You’re sister might even be able to claim herself head of household and your mother as her dependent if she pays more than half your mom’s support for the year-including for housing.

It seems like you don’t qualify based purely on the 6 month rule-your math on your living expenses seems a bit off to me and probably would to the irs too.

Just try to remember to take into account things like bills and such. Internet, phone, car, car insurance, Health insurance, the cost of housing and the associated utilities. Unless you live in the middle of no where, average rent is about 780 a month, average internet 60, average phone 70, average utilities 170 and so on. That doesn’t even cover a full 4 months with just those 4 numbers. So be careful if you have to move in with your mom again because you’ll very likely be someone’s dependent-be it your mom or your sister. And let the grandma know you’re grateful because she’s surely saving you a lot of money on rent and utilities.