My purrrrrrrrsonal reaction to that vet is rubbish and not likely to be liver at all, would guess at thyroid problems most likely.
I would advise you sister to find a better vet urgently and get some condensed milk.......carnation do it in tubes,,,,,,mix a teaspoonful with water and hope the cat will drink it. luke warm is best. do this once a day for a couple of days and hope the system will work again. If it doesnt work will need vet help.
I have had two cats over 18yrs and still have one and Napoleon was 17 going on 18.
Fits are not common in a cat of any age and it needs the basic blood tests done for kidneys and thyroid at least, plus advise for the fits.
I can help on how to deal with them but the cat needs to see a good cat vet now and first.