Some languages have different words ti express you in singular and in plural. In the past, English too had such a differentiation. In Old English, the primary word for "plural you" (nominative case, used as the subject) is ge (pronounced /jeː/), while the object form was ēow. As the language transitioned to Middle English, these evolved into ye (subject) and you (object), often used to address multiple people. 

Now, there is no reason to believe that Sarah was not privy to the conversation in Gen 22:2. After all, she was Isaac's mother and had been privy to the conversation between Abraham and the Messenger of God, with the latter announcing would be arrival of their son. And, Sarah  was, at that time, Abraham's legally wedded life vis- a-vis Hagar and had borne Isaac their first son for him. How could God keep her away from such an important announcement and expect Abraham to smuggle Isaac out of home and Sarah to certain death. So, once we consider that Sarah was privy to God's command to Abraham in Gen 22:2 , all doubts vanish