Some languages have different words to 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(*Courtesy: Google AI) At present, English language has only one word 'you' to express either one person or more than one.
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 in advance, the birth of their first- born son (Gen 18:10).
And, Sarah was, at the time of the test, Abraham's legally wedded wife vis- a-vis Hagar, and had borne Isaac their first-born son .In Gen 22:2, God names Isaac , the son to be sacrificed. How could God keep Sarah away from such an important announcement and expect Abraham to inform her ? She wouldn't believe him and would alert Isaac. That meant Abraham would have to 'smuggle' Isaac out of home and from Sarah to certain death. God would be the last to deprive a mother of the right to know what was going to happen to her only child .
Also note the expression " whom you love" . In NT, John the Evangelist uses the expression " one whom Jesus loved " to refer to himself. That expression would become pertinent when Jesus handed over his mother's care to John. As an OT counterpart of John, Isaac was looked upon by Abraham and Sarah as their legal heir, which fact was corroborated by the expression "whom you love ".
So, once we consider:
... that Sarah was privy to God's command to Abraham in Gen 22:2 (just as she had been, in Gen 18:10), and that God meant both Abraham and Sarah by the salutation 'you/your' whose heir Isaac was poised to become...
......it become clear that the expression 'Only Son whom you love ' refers to Isaac.