She wasn't really related to Laura. Amy was the biological daughter of the Vinings. The Vining's first child died at birth, and Laura was switched in the hospital. Laura was raised by the Vinings and when Amy was born, she was considered Laura's little sister. Leslie finally tracked down Laura and reclaimed her. Amy then left the Vinings and went to live with Leslie and Rick Webber. Laura and Amy did consider themselves sisters, however. Shell Kepler was on contract until the turn of the century, and then she went to recurring. I'm not sure when she stopped making appearances. By the time she went to recurring, she had almost no air time anyway and was usually only seen in full cast scenes, like the Nurses Ball, the Christmas Party in the children's ward, weddings, funerals.