How to trace who sent email from yahoo account; family member hacked email account

A user's iPad shows a message sent but the user did reply or respond to the email received, and did not send a reply back to sender. The user believes that someone accessed their Yahoo mail account, sent a reply, and signed sent message from the user's iPad. Can this be traced to find out who sent and accessed the user's Yahoo account? The user believes it was another family member, but would like to have visual proof. What steps can I take?