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.