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I am so sorry you are having to go thru this...
Without examining the cat and viewing bloodwork...I don't have an educated guess as to the cause of death.
Siamese cats do have issues cardiomyopathy which can come up acutely and cause death...
If the severity of weight loss and muscle atrophy...I find it hard to believe that the bloodwork was normal...but it is all hindsight.
Central nervous system infections can do this as well.
So sorry I can't help more with the information.
First, the eye description doesn't reflect anything. This is common in most animals as they are passing...It usually come from the decrease in oxygen getting to the brain and ocular tissues and you really cant put much into that unless it was a clinical sign several days before dying.
IF there was a disease the affected the nervous system like toxoplasmosis or such, you can get the sibling tested.
Cardiomyopathy alone does not cause what you see, but failure to correctly work affects all the organs which can cause the muscle wasting...which is why I question the bloodwork.
Immune mediated diseases can occur in pure bred cats as well.
Vamvakias is Greek ...Kansas for 6 more weeks !
Again...the pieces are there,but I am so limited online.
I am sorry your family has experienced this!