I Start Counting (1970)

Jenny Agutter plays Wynne, an adopted 14-year-old girl who has a crush on her 32-year-old stepbrother, George, played by Bryan Marshall. While spying on George in the bathroom, Wynne notices George has several scratches on his back, and finds a bloody sweater she made for him that he threw in the trash, which leads her to suspect George of being the serial killer of several local teenage girls, who is still at large. Despite this belief, Wynne continues to have romantic sexual fantasies about George and dreams of marrying him when she comes of legal age.