Droopy ear.
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Yes, ear mites or an ear infection. I'd lean towards an ear infection because I think when a cat has mites, it's usually in both ears? Either way, he needs to see the vet. Ear infections are painful and they get worse quickly. The sooner it's treated (with antibiotic drops), the easier and quicker it is to clear up.
Looking at two of your photos, I definitely recognise that slight ear-fold Gatsby had when he had two ear infections as a kitten. He was shaking his head too (a sign of pain) and scratching a lot.
The vet will almost certainly be open on Saturday, like most of them are, for people who work during the week. But tell your sister to give them a ring quickly, as Saturdays are usually a really busy day for vets.
OK, I'm adding some photos to give you a better look.



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