Ah, Aunt Polly: tenderhearted, loving, but usually misguided Aunt Polly. She is the caretaker of Tom Sawyer (after Polly's sister, Tom's mother, died) and is always desperate to understand Tom. Her favorite charge, however, is Tom's brother, Sid. Aunt Polly is always asking Tom to look to Sid in regards to how to behave.
Aunt Polly is usually seen either quoting scripture, asking Tom to be more like Sid, warning Tom to be more civilized, or hitting Tom over the head withe her thimble. Tom is not a fan of this type of treatment, so for all of Aunt Polly's tries, ... she fails to win Tom over.
Aunt Polly is also the glue that cements the rivalry between Sid and Tom as well. Polly is always trying to figure out Tom's doings. Tom is always trying to give her some kind of convoluted explanation (and usually succeeds). Sid prides himself in helping Polly note the inaccuracies in Tom's descriptions. (Thus, Tom gets another thump with the thimble.) In my opinion, Aunt Polly's relationship with Tom is summed up here:
In another moment he was flying down the street with his pail and a tingling rear, Tom was whitewashing with vigor, and Aunt Polly was retiring from the field with a slipper in her hand and triumph in her eye.
But of course, we KNOW what happens in regards to Tom's whitewashing the fence. Aunt Polly never seems to have the upper hand!