Sticky situation

Hi,
i am cat sitting my sister's 15 year old cat, Mickey
well, actually, Mickey is more or less "mine" now, as long as my landlord doesn't find out (i am only allowed to have one cat, and i have a 3 yr old of my own)

my sister is paying for cat food and litter and i buy treats and such.

Mickey was over weight when i got her and she has lost weight which is good

(she overate at home due to competition with the dog- which is why she is staying with me, she wasn't happy at home, my sister's dog is also elderly and has become a bit of a problem to handle- plus he is now always trying to "eat" the kitty! so she is safer here!)

well, the thing is, although her appetite is good (and has been right along) she seems to still be losing weight

i could be just paranoid about it, i don't know, but i want to take her to the vet and have her checked out just in case

the thing is, my sister can not afford any vet bills right now, which is why i want to take her to the vet and not tell her that i am-
and of course i'd pay
Two reasons- one, i know she can't afford it
and two- she(my sis) is under so much stress right now, i don't think she could handle anything else, and i may just be overreacting, after all, i am responsible for a cat who technically does not belong to me

MY QUESTION- DO YOU THINK IT'D BE OK TO TAKE HER TO THE VET WITHOUT TELLING MY SISTER ??
(or asking her persmission?)

i mean, is there anything bad that could happen?
i guess the only problem would be if the vet says she has some serious (that is, expensive to treat) problem- then i'd have to tell my sister, as the decision on what to do would have to be hers (i am on a low fixed income- i have emergency savings in case my cat gets ill, but i do not have enough for two cats)

what should i do?
i am going to try to weigh her later today (by weighing myself then holding her and seeing what the increase is- its not too accurate but maybe i can figure out if she really is losing more weight)

i know cats are not supposed to lose weight quickly, even when they have a wt related health problem (i had a diabetic cat before and was told that she should lose wt but very gradually!)

help!

One of the most common reasons for an older cat to lose weight and still have a good appetite is hyperthyroidism, which means her thyroid is overproducing thyroid hormones and this leads to weight loss no matter how much the cat eats.

This could be one of a handful of reasons why your cat is losing weight and giving her age it would be best to have her looked at by a veterinarian.

This is just my opinon but if I were you I'd bring the cat without telling your sister at first because it may very well be that the cat is fine and you might cause your sister some more stress she could deal without. Afterwards you could always tell her you brought the cat in for a senior check up or if something is found and can be taken care of I'm sure she would be relieved to know the cat is going to be ok.