The hair loss on the curly was sounding familiar so I went hunting for info - apparently, alopecia is not uncommon in the breed so it’s very possible that the curly’s hair loss has nothing to do with your mare’s. (Or it might. In any case, fingers crossed that an easily-treatable cause is figured out quickly!)
Hair loss in curly horses:
One of the mysteries of curly coated horses around the world is the problem of alopecia (abnormal absence of hair).
Alopecia in curly coated horses can range from thin or absent mane and tail hair, to bald patches on body coats, and can also include an almost complete absence of hair. To make matters even more complicated, we now know that various populations of curly coated horses have multiple causative genes.