Would it still be "her house" or more of a grandma-lives-with-us-situation?

My sister lived with her kids and husband under the same roof with my mother and the situation was extremely complicated because my sister always were to be a guest in a way.

You know, with my mom being a single mom, I think there is something of an interdependency between her and her youngest child (my sister). Possibly even a codependency. That is to say: there is no real power struggle between them. They accept their roles as mother and daughter pretty well. They share their space effectively because they both accept their roles. If my sister were to exert more of a commanding influence on the household, there could be conflict, but so far this has not been the case.