So small head canon, Maddie's several careers before her and Jack's current position as Danville Beach's lifeguard, consist of her working for that photo company in the Dexter's Laboratory episode 'Photo Finish', so yeah with Maggie being her sister, she technically fought her nephew that episode. She only saw Dexter in very sporadic occasions.

Later on, she quit upon realising the company's true intent (something about one big privacy invasion scheme), gained some weight and developed some great skill in violin playing, which got her a position in Danville Symphony Orchestra.

And of course, while in this orchestra, she started loosing weight, growing taller than even her sister, as tall as Jack and Vlad, and became a lifeguard.

Maggie and Debbie really do have the "3" tushies.

Let's gather round the campfire

and sing our campfire song