Oh no. :( Sorry to hear that. I have A LOT of experience with sinus infections, one that actually became strep as well. Not fun. There was one year where I kept getting sick (like Katie) and I was just not getting any better. By the end of that year, I think my immune system was very weakened (illness + antibiotics not always so great on the body). So I started doing a couple of things (I'm no doctor but this might help):
-regularly rinse with neti pot or rather the neti bottle (even a doctor recommended I do this)
-take vitamin supplements - for me I took vitamin C and calcium + vitamin D
-exercise regularly - I know that's hard right now, especially with this stupid cold weather, but if you guys can go out for daily (or almost daily) walks, that would help
-finally get some rest whenever possible

I so hope she feels better soon.