The brothers and sisters were taught how to do it by her.

句意：她教兄弟姐妹们怎么做.
你给的原句是被动语态,by后边是动作的发出者,也就是she(主语,用名词主格),动词是teach（taught的动词原形）,教谁?the brothers and sisters,教什么呢?教的是how to do it.好了,句子逻辑关系理清楚了,现在开始变：
She taught(过去式) the brothers and sister how to do it.
完成.