I don't know the ins and outs of your relationship so I can't answer that for you.

Like I wrote in my post earlier: you are firm but fair/calm.

Examples:
She addresses you with respect or you do not engage with her.
You don't cater to her when she treats you like crap.
You reinforce the expectations you have while she is living under your roof
You stop making excuses for her (e.g. she is suffering inside) which only serves to enable her to continue her appaling behaviour

Etc.