It's not unusual for some pets to mark new things in the home.
He doesn't do it any more but between 1-2 yrs of age my dog would pee on any new blankets on the floor or new pillows in his bed.

Like you, I think she should have the cat checked for any health problems.
Or......maybe the cat 'has' reached its limit in changes! Still needs a checkup tho!

I read somewhere (maybe the Flooring forum) that there is something in new carpet tape or glue that attracts cats. Sorry for lack of specifics, but it may be something in the carpet rather than something in the cat.

I found this article doing research on my cat with CRF, apparently there was a study that new carpet can smell like cat urine, to cats:

http://cats.about.com/cs/cleaningmesses/a/newcarpeturine.htm

Here's a snippet:

"In lay terms, tiny microorganisms (bacteria) find a home in the latex backing of the carpet, and during their growth produce butyric acid, which is a weak acid that will not harm the carpet, but does have a decided odor that to some humans (and cats apparently) smells like cat urine. This particular bacteria appears to be anaerobic, which means it is capable of growth outside of oxygen. The standard disinfectant used in carpet manufacturing apparently does not affect these microorganisms, and any new disinfectants will need to be tested to ensure they do not harm the latex backing."

A check at the vets may be in order too, as inappropriate elimination is often a sign of a heath issue. Good luck to your sister!