sit by and watch 作壁上观

“作壁上观”，汉语成语，原指双方交战，自己站在壁垒上旁观（watch the fighting from the ramparts）。后多比喻站在一旁看着，不动手帮助，可以翻译为“sit by and watch，be an onlooker/by-stander”。

例句：

他们吵得不可开交，你要去制止，不能作壁上观。

They are quarreling fiercely. You should go and stop them rather than sit and watch.