What is in the letter from Aunt Polly? Why doesn’t Aunt Sally see it?

The letter was to Aunt Sally. Knowing Sid is at home with her, Aunt Polly becomes suspicious. She writes two letters to Sally asking her to explain what she meant by Sid arriving with Tom. It doesn't get read. Just as Silas is about to give the letter to Aunt Sally, they see Jim, in a woman's dress, Tom on a mattress, and the doctor coming in the distance. This provides enough of a distraction not to see the letter.