Maureen is the youngest of the family. We know little about her, because it seems that Jeannette doesn't understand her baby sister. Maureen's the oddball of the family. She has nightmares. She joins a cult. She stabs Mom, gets put in a mental hospital, and moves to California all by herself. Considering the fact that she grew up sleeping with rats, we'd say she still turned out pretty well.
Maureen shows us that different people growing up in the same environment will all turn out differently. If Jeannette were telling this story as if it were "Goldilocks and the Four Bears," Lori might be the smart one, Brian the strong one, Maureen the unusual one, and Jeannette the just right one.