Values Dissonance: Shuuko fears that, if she stays with her daughter Misaki, some traits of her will bring said child the scorn of others due to having "a useless mother". In the manga, she's cripplingly and horribly shy; in the anime, she's an Ill Girl in a wheelchair, AND cripplingly and horribly shy. This comes from the very complicated view of human relationships in Japan.
Subverted in that Shuuko is mostly accepted by her peers, even her own sister, and is the champion of Angelic Layer, so it's not like anyone hates her for her disability. Her sister even outright says that there are lots of people in wheelchairs who are good parents who have even worse diseases than she has.