The Kirlian Witness (1978)

Laurie believes that it's possible to communicate with plants via telepathy and devotes all her time and love to them. Her plants do warn her of her sister Rilla's new boyfriend Robert. When Laurie's found dead on the street under her balcony one morning, Rilla doubts that it was an accident. Only a plant was witness that night, so she tries to find a way to learn the truth from it.