NORTHVILLE — At 91 years old, Bethlehem resident Ruth Mendel is part of a shrinking group of survivors of the Holocaust. Her story: she escaped the genocide that killed six million Jews across Nazi Germany-occupied Europe more than 80 years ago. More than half of her family was killed. During a lecture at Northville Central School, she recounted all the details — moments of heartache, bedlam and hope. As Holocaust denialism persists, she said, bottling the memories up isn't an option. "If there were no need for it, I wouldn't do it," said Mendel. While living nearly an hour away, Mendel is no…