This could possibly be Wes Craven's 1978 TV movie called
"Stranger In Our House".A young girl's cousin goes to stay at her house and suspects her of being a witch. Her suspicions turn out to be true.
http://www.imdb.com/title/tt0078330/It also goes by the name of
"Summer Of Fear", it stars Linda Blair.
Another possibility might be "Sarah's Initiation" but it's a bit more like a "Carrie" rip-off.