I would take the remaining kid (or it's poop) in to a vet for a fecal / well baby check.
I'd also probably give her a selenium injection, just for grins.
Has she been vaccinated w/ CDT and boosted?
We lost kids our first year to cocci, and because they were our first, I didn't really know anything was wrong, since they ate, played, etc.
Once I started treating them and saw how kids were supposed to act, the difference was amazing.
sorry about your loss but you need to concentrate on your living kid to prevent another loss...I would get the healthy kid started on a feed with some sort of coccidistat, decox,monensin, whatever you like....I tend to agree with RF a kid that overloads on cocci doesn't have much of a chance...besides even if it is something else the feed additive can only help th youngster and you will be able to take the cocci cause off your worry list...
Keep in mind also that if they already have a heavy cocci load, the feed alone will not 'cure' them, they will need treated for several days with something to kill it off.
I use dimethox for 5-7 days to treat.
The medicated feeds will prevent cocci if they eat enough of it to keep the levels of medication high enough to do so...it will not KILL cocci already running rampant in their system.