Bonnie's Kids (1973)

Sisters Myra and Ellie have finally had enough of their miserable, dead-end lives. When their stepfather Charley (the titular "Bonnie" being long dead) tried to rape Myra, Ellie ventilates him with a shotgun, and the pair run off to their wealthy uncle's mansion in El Paso. From that point on, the two undergo a transformation in their personalities, and start to enjoy living their lives on the wild side.