Beatrice, Ruth's unstable mother, makes money from taking sick people into her household. It is not a lot of money. It even makes one wonder whether Beatrice could possibly make the effort to find something less traumatizing and serious with which to make a living.
In act 1, we get a glimpse of how this living arrangement affects Ruth. One of the boarders, Mr. Mayo, is a man who suffers from a brain tumor. It is implied in the play that Ruth checks on Mr. Mayo and, one night, finds him dead. The event is traumatic enough to still affect Ruth. She says,
He's after me! [. . .] It was the dream, with Mr. Mayo again. [. . .] Well, they say I came out of my room . . . And I started down the stairs, step by step . . . and I heard the choking and banging on the bed, and . . .
She is traumatized to the point of having nightmares in which Mr. Mayo chases after her. The nightmares set off her seizures, and her condition becomes a never-ending, vicious cycle where nothing really gets resolved. After all, Beatrice essentially tells Ruth to not let it get to her and go on about her business as if the nightmares and seizures were part of their "normal" life:
You're not going to let yourself go, do you hear me, Ruth? You're not going to go!
Beatrice essentially puts the blame on Ruth for setting off her own seizures due to traumas brought to the home by Beatrice herself.
This said, it is arguable that the addition of sick people to an already toxic living environment is tantamount to another layer of negative energy in the household. It means an additional source of stress for the girls and Beatrice alike. It makes the situation in the home all the more oppressive and conducive to chaos.
This should not be shocking, since Beatrice thrives on chaos; as a toxic person, she needs this negativity as part of her pathological behavior.