After reading Paul Zindel's The Effects of Gamma Rays in Man-in-the-Moon Marigolds, the reader is left with little to no doubt that Ruth Hunsdorfer's life has been forever altered by her mother in the same fashion that the Man-in-the-Moon marigolds become negatively altered by the toxic exposure to radiation.
Ruth's demeanor and dynamics are the result of what is known as a hot/cold connection to a material figure which is neither safe, nor loving. As an elder sister, Ruth has experienced her mother drunkenness, her fits of rage, her constant negative remarks about life, and the insults that Beatrice throws at her. Notice how, in Act 2, when Beatrice tells Ruth that she has to stay behind taking care of Nanny instead of going to see Tillie at the Science fair, Beatrice lays it down quite heavily when she says to Ruth:
Somewhere in the back of this turtle-sized brain of mine I feel just a little proud! Jesus Christ! And you begrudge e even that, you little bastard!
Treatment like that is capable of dissolving whatever foundation of safety and love that Ruth may have been able to build.
Ruth DOES experience joy. She even experiences pride. Both of these things occur as she hears of Tillie's nomination for the Science Fair. We see the hope and joy in Ruth's reaction as she proudly keeps repeating the words:
MY sister! MY SISTER!
However, Ruth's world is not one which is accustomed to perennial joy, nor is she mentally prepared to experience it. As a result, her naturally-built pathos moves her to ruin the moment by picking on Beatrice's looks, and by telling Tillie about the things said about Tillie and Beatrice behind their backs.
She also has a problem negotiating her role in the family, since her mother consistently uses her or enables her but does not support nor love her. This is why Ruth has an issue over the ownership of the rabbit, over her mother's feelings for her compared to Tillie, and over her participation in events such as Tillie's Science fair.
In the end, there is little that indicates whether Ruth's last seizure (this was the worst of them all) will render her disabled, whether she will wake up from it, or whether she will continue a normal life after her mother did the ultimate act of hatred and betrayal in killing her beloved pet. There is no way to know. However, the psychological and emotional abuse that Ruth suffers at the hands of her mother leave no room to doubt that she will, indeed, have a problem in terms of feeling fulfilled, or successful; after all, the source of all her insecurities and fears is the same person who is, by nature, supposed to nurture and protect her. There is very little hope for Ruth. She has been engulfed by the black hole that is her mother for far too long now.
Ruth has a job and this is the only indication of her success (getting the job) but we don't know if she's good at it. We know that she lives in an environment where she is not encouraged. We don't know what her passions or abilities are because the dialogue between her and Beatrice only reveals resentment. Beatrice blames her daughters (and fate; anything to put the responsibility on people and events other than herself) for her misfortune. This relationship sustains itself because neither side is a positive influence on the other. The only time that Ruth expresses self-confidence is when she rails back at her mother. Notable examples are when she insists on going to the science fair and when she threatens to kill Beatrice after finding the rabbit. But these are reactions to Beatrice's tyranny. Tillie is the only one of the three who experiences joy because she encourages herself and receives encouragement from her teachers. Ruth probably never got this kind of encouragement outside the home. Beatrice killed Ruth's spirit by creating an environment where it was impossible for it to thrive.