Zweierlei Maß (1964)

When the Duke of Vienna takes a mysterious leave of absence and leaves the strict Angelo in charge, things couldn't be worse for Claudio, who is sentenced to death for premarital sex. His nun-in-training sister Isabella is very persuasive at pleader, but when she goes to Angelo, instead of freeing her brother, he offers to save his life if she sleeps with him. Isabella's only sympathetic friend is a priest who is actually the Duke in disguise, and he has a plan.