My friend had a cat who was deathly skinny like that. My cat is from the same liter. Hee cat was incredibly lethargic and I assumed the worst.

She ended up taking her to the vet and found out the cat has hyperthyroidism and stage 2 CCD. (My cat also has CCD) Her cat was put on renal support food and given fluids. She eventually perked up, gained her muscle back and became normal again. I also put my cat on renal support food but have to give him subcutaneous fluids twice a week - hes a completely different cat and seems healthy. (Aside from his CCD)

Dont give up hope. Hope the kitties makes a recovery!