Is it illegal for someone to view my private Instagram through someone else’s account? I recently found out that someone who I did not allow to follow me sent me a screenshot of one of my posts. I know that they didn’t hack me, so the next thing I go to is that someone is allowing them to see through the account I did allow to follow me. Is this against the law in any way?
In the US, there are no such laws which restrict what a person can see. You have no recourse against the sender. You might be (remotely) able to get the service to expel the friend whom you suspect of being instrumental in the sender seeing your material, if whatever the friend did was contrary to the terms of service. Read the section "How You Can't Use Instagram" to see if you think your friend did any of those things. You might think you would be able to sue the service for breach of contract, except that the service never promised that they would protect you from people who might see what you have posted. Also, if you read their TOS, you will see that you actually can't sue them (arbitration clause) Realistically, there is nothing you can do other than to retract "following permission".