Well first, I would separate her for a bit because she'll give the others this - and you've had one apparently already that was sick.
On the sinus flushing, you can use a home-made saline solution. Ducks are way more hard to aspirate because of the way their mouths/airways are set up. Hold the duck in your lap, clean the nares out as well as you can. Use a syringe (without the needle) and stick it into the nostril pointing down and towards the end of the beak. Squeeze the warm saline water gently to flush the gunk out. POint the tip of her beak down gently as you do this. She's going to struggle, but just be gentle with her.
You want to make an "isotonic" solution - one that isn't more salty than the normal body fluids are. So it doesn't dry out the sinuses nor does it draw out anything - just gently cleanses.
1/4 teaspoon of walt (table salt, unless you have canning salt)
1 cup of warm water (distilled if you have it, or filtered)
1/4th teaspoon of baking soda
Keeping the gunk out of the nostrils will help healing.