Who is Erin Graham from Mortal Fear and what is their importance?

Erin is Drewe's sister. Erin left school, became a model, did drugs and had sex with many men, leaving the field very soon after her career began. While Drewe was finishing medical school and Harper was living in Chicago, Erin shows up at Harper's apartment. They have sex almost constantly for three days, and Erin ends up pregnant with a daughter, Holly. She soon marries Patrick with the understanding that he is not to ask about Holly's father. When Erin's marriage deteriorates, she tells Harper that she plans to tell Patrick and demands that he tell Drewe. For some unknown reason, Erin then shows up at Harper's house and encounters the killer, Berkmann. She dies but also kills Berkmann's wife, Kali.