The eldest of the Fitzgerald boys, Sweyn is going on twelve. While he still plays with Tom and J.D. quite a bit and even teaches J.D. to swim, he has one foot in the adult world. He's not part of Tom's plan to charge kids to watch Mr. Harvey dig the cesspool because he's "helping at the newspaper office" (1.39) and because Papa leaves him at home to keep order while the other kids watch, not to have fun himself.
Because of his positioning as Big Brother, Sweyn is never taken in by Tom's schemes and is often the one who steps in when Tom steps over a line:
"For gosh sakes, T.D.," he said as if disgusted with Tom, "can't you let anybody have a nickel to themselves without trying to connive them out of it?" Then he looked at me. "Go put the money in your bank, J.D., before the Great Brain figures out a way to take it away from you." (3.63)
Sweyn leaves for boarding school in Salt Lake City at the beginning of Chapter 7, and without him around to apply the brakes, Tom really ups the ante on his schemes, trying to get Mr. Standish fired and get his hands on Andy Anderson's erector set.