Who is Rosemary and Jack from The Memory Keeper's Daughter and what is their importance?

Rosemary was a pregnant teenager living in an empty, decrepit house in the country that belonged to David Henry. When David, distraught, visits his childhood home and finds Rosemary there, she very capably ties him up while he sleeps, just in case he is dangerous.David is with Rosemary when she is in labor and, when he leaves Norah, he takes Rosemary with him to a duplex, where she and her baby, Jack, live next door rent-free.