(Darke County, OH)--A Darke County teenage brother and sister were killed when their car collided with a semi Monday night. "This is a terrible tragedy at any time of the year. But, it seems like, approaching Christmas, it's certainly that much worse for the families that are involved here," said Chief Deputy Mark Whitaker. 17-year-old Hudson Nestor and his 13-year-old sister Katelynn were pronounced dead at the scene of the crash on Neff Road a few miles northeast of Arcanum. The cause remains under investigation. The Nestor children were students in the Franklin-Monroe system.