Claiming my S\O mother who has been living with me for a few years.

Claiming my S\O mother who has been living with me for a few years. I pay all the rent utilities and food expenses all house hold expenses. She was working 3yrs ago but hurt her self her SDI ran out.  I dont mind her living with us. Me and my S\O been together for 11 years her mom living with us for 8 total.  Or should my S/O claim her as a dependent.  I would or we would claim her on the upcoming tax year. Also Can I claim her as my dependent on my W4 through out year.  She has zero taxable income.