Another company does the same thing: Dark Horse.
Also, Antech Diagnostics employs specialists for phone-consults. However, your sister-in-law may need additional education to pursue that type of career.
If she enjoys academic stuff, perhaps working from home with one of the veterinary journals - as an editor or in some other capacity - there's many publications out there.
Not sure how the national animal poison control consulting vets work, or from where, or what - if any - additional degrees are required - but that might be something a DVM could do from home.
Relief work can be done on a per-diem basis, or, as someone else mentioned, weekend vaccine clinics. I'm sure there has to be an emergency practice somewhere in the vicinity that might welcome a part-timer to fill-in a shift every now and then, perhaps. Granted, that's not at-home, but it could be off-hours or more flexible in hours/days/nights. Say, working one overnight shift a month, or twice a month, etc.