Sorry for your loss. The turk wasn't quite 3 months old, yes? Do you know how your cousin was maintaining them (inside barn/coop/any external heat source)? Were they out in the rain (I know we got about 0.5" up in NE part of County)? One thing that comes to mind is weather related stress (went from 56° down to 22° F with a pretty brutal wind chill). Younger turks are more susceptible; and they were recently moved to a new place (also stress).
Observe your survivor closely: standing - often on one leg with other leg and foot tucked into feathers/hunched up in corner of coop, unfeathered skin very pale - just looking `cold'? - might want to cage it up in `mudroom' (or anywhere it is slightly warmer) some cornbread/crushed hardboiled eggs are useful. Check droppings (bloody/yellow/green/clear liquid/etc), listen for any respiratory distress (wheezing/sneezing/exudate from nares/around eyes/swelling of face/head). Were they out foraging in run while it was raining?
Vet Path Lab at the University performs necroscopies (lab testing):
http://vmdl.missouri.edu/VMDLSchedOfFees.pdf
Our Vet, Dr. Debbie Leach, has performed surgery on one of our toms and has treated our roo. Her undergrad degree is in poultry science with a Masters in poultry genetics (will want to examine the poultry - no hypothesizing over the phone).:
http://www.myzooanimalhospital.com/ Just a good chook/turk doc to keep on file.
Good luck! and hope survivor grows like a weed!