Right,
Which is the one I provided in the very first response.
I also stand by my recommendation which was seconded by Reinz to buy THE COMPLETE CARPET PYTHON . There is some very specific information about diamonds in the book.
Personally I would invest in the largest cage you can, within reason, and provide wide a range of climate zones.
Provide some "climate levels" as well. Perches, a "high hide box" (http://ball-pythons.net/forums/showt...ing-IJ-Carpets), a floor hide and some moss coupled with cypress bedding for substrate, and as mentioned here, consider using a radiant heat panel (RHP) to heat the cage.
Here is a RHP setup with perches. This is a coastal mix/mutt. There are floor hides, and perches. The perch directly under the panel obviously has the warmest temp in the cage. The other areas all have slightly different temps.
Again, pure diamonds are a different bag of tricks. As I understand it, once they are crossed with another type of carpet their specific requirements tend to breed out diminishing, or totally negating the exacting husbandry the pure specimens require.
Your sister may or may not have been doing well with the animal. Reptiles tend to exhibit illnesses that stem from long term errors on our part and they don't often show up immediately. I'm not going to speculate there, as we are moving toward getting you on the right track.
You have some time to fix things, but not a lot. Handling should be extremely limited and only done when absolutely necessary. The same for feeding unless there is an obvious need for it which should not be the case with an older snake.
ANY stress the snake feels will compound the problems and the downward spiral will continue.
STUDY the care sheet, most importantly the temps. Buy that book too. Don't discount an experienced reptile vet if things don't improve.
Good luck.