The Strange Ones(1950)

Original title: Les Enfants terribles

Elisabeth and her brother Paul live isolated from much of the world after Paul is injured in a snowball fight. As a coping mechanism, the two conjure up a hermetic dream of their own making. Their relationship, however, isn't exactly wholesome. Jealousy and a malevolent undercurrent intrude on their fantasy when Elisabeth invites the strange Agathe to stay with them -- and Paul is immediately attracted to her.