Shelly and Dotty Donahue

Im sorry if this has been mentioned before, but has anyone else been confused by the Donahue girls and how they're names seem to change? In the episode where Brick rakes the leaves, Frankie says that Molly Donahue has a little leaf raking business. Who is Molly Donahue?

Also the episode where Frankie gets stuck with the Christmas reefs, the younger sister says "Shelley taught me how to use a glue gun", yet it's later established that the older sister is Dotty and Shelly is the younger sister.