Well, lets see.
If smol Jacob can lift about 40x his weight, I’d say a regular borrower averages 10 to 15x. The brothers, being determined to keep physically fit, are likely around the 12 to 13x range. Once they get better nutrition, they’ll easily hit around 17 or 18x their weight in strength, if not more.
Really, the questions is: how long does it take the sprites to realize they should just start flying and lift the brothers off the ground so they can’t get any traction?
With wings, they’d just go with that route. Lift up the brothers while Jacob watches, unable to hide his snickering (Dean will get revenge on him, somehow). At least they’re good at dangling from a rope.
If they weren’t allowed to use wings except maybe to balance themselves, it probably wouldn’t take much for the brothers to knock several knights over, one by one. Sprites aren’t nearly as strong as one of the brothers. Poor guys would dig their heels in the best they could though. Then maybe have a few sparring matches with pine needle swords to make up for it.
(thanks to @torchmlp for the reference of smol Jacob’s strength)