According to a 2013 article in the Daily Herald - a suburban Chicago news source - David Newell's real wife, Nan, played the part of the maid of honor in the wedding film. He refers to her in this episode as "Betsy's sister."
Mr. McFeely mentions that he and Mrs. McFeely have been "married for 45 years." This episode aired in 1981 which means they would have married in 1936. Estimating that Mr. McFeely was 20 when he married, theoretically he would have been 85 years old when Mister Rogers' Neighborhood ended in 2001.