Roy did his duty by his younger sister.【误译】罗伊的妹妹已替罗伊尽了义务。

【正确】罗伊对他的妹妹尽了义务。

说明： 本例的 by (介词) 不能作"由"解，而应解作"对",此处的 by 与 to 同义。