This episode reveals Sue Ellen's backstory on how she came to Elwood City. However, she has been seen in many episodes in Arthur's 2nd grade flashbacks and even in "Arthur and the Real Mr. Ratburn" where their 3rd grade school year begins.
In "The Perfect Brother", this is the second time underwear is seen in the series and before "Arthur's Underwear" that aired later during the show's second season. This is when the Brain dumps his clothes out of his duffle bag in his room that's revealed to be messy despite being neat at Arthur's house.