A Republican state senator will take over.
From the Philadelphia Inquirer:
Lt. Gov. Catherine Baker Knoll, 78, the first woman elected to that office in Pennsylvania history, died today after a battle with a rare form of cancer…
She will be replaced as lieutenant governor by Senate President Pro Tempore Joe Scarnati (R., Jefferson).
Drew Crompton, a spokesman for Scarnati, said a decision about a swearing-in ceremony will be made tomorrow. He said Scarnati will retain the role as president pro tempore as his predecessor Robert Jubelirer did in 2001 when Lt. Gov. Mark Schweiker became governor after Gov. Tom Ridge left to become Homeland Security director.