Abby Cassidy is Cate's younger sister, and Lux's aunt, as well as the daughter of Grant and Laverne Cassidy. She somewhat into healthy living, is a Yoga instructor, and is involved to some degree with Kabbalah as she wears a Kabbalah bracelet, and also gave one to Baze. She has had a crush on Baze since highschool, and has a secret fling with him behind Cate's back after she meets Lux. Lux catches them in bed, and is upset that Baze would sleep with Cate's sister, but decides not to tell her about it. As it turns out the fling becomes more serious, and the two become a couple - which Cate finds out for herself when she attends Abby's yoga class and Baze is there with the Kabbalah bracelet on.
Her last appearance is at thanksgiving dinner, in which she accidentally tells Baze's dad about his relationship with Emma. She makes no more appearances after this, being absent during Lux's graduation in the finale.
Unlike Cate, Abby appears to have a better relationship with their mother, even still living with her at age thirty(as she reveals she was fourteen when Cate was sixteen and pregnant making her two years younger). This could stem from the fact that Laverne had her intentionally, and unexpectedly had Cate. She also did not want Cate. This could also be why Cate has a distant relationship with her as well.