Who is Helen Meredith - Rhiannon's estranged sister from The Broken Bridge and what is their importance?

Helen Meredith is Ginny's best friend Rhiannon's estranged sister who lives in Porthafon. She has a job, worldly knowledge and lives far enough away so as not to be a threat to Rhiannon. This is unlike Ginny's half-brother Robert who invades Ginny's territory when he moves in with her and her father. Helen is like an older sister to Ginny; Ginny runs to Helen first for clarification of the prison rumors, then to her for help in getting the truth out of Joe Chicago. Helen is fond of Ginny and vice versa.