Let's talk neuro conditions.
Sable (pictured) and Artic her littermate both have a neurological condition that makes them very uncoordinated, much like Cerebellar Hypoplasia - but based on my research, CH is very uncommon in dogs, and it would be so rare that littermate would both be effected. Sable has more coordination than Artie, she can get up on all 4 legs and walk, it's very jolting movements, but she can. Artie can not stand up on his back legs, he has motor function of them, but can not stand/walk.
I know I could go see a neurologist and pay a huge amount for an MRI - but this isn't a fixable condition, I know that much, so why spend the resources, when that money could be used for another dog. (I run a rescue)
So, my question is - what are the possible conditions they are dealing with? Their mobility does seem to be worsening over time.