As it is her house, are you paying her rent or utilities?
If not, do you plan to? If so, then you could have a rental agreement in place and that means she can't come into your place anytime she wants, just as a landlord does not have the right to come into a tenants place at will.
If you are not paying any rent, then I am afraid yes, she can come and go at her choosing. That would annoy the crud out of me.
