Hi there, and sorry to hear your kitten is poorly
I agree with what Aud's Mum has said - you need to put both kittens on a diet of meat only, preferably raw, chicken is great but any other meat will do fine too.
Cut it up in small pieces for them and let them eat as much as they like. If they won't eat it at first, try putting a
little bit of the Whiskas on top to tempt them to eating. Also start offering them the tip of some raw chicken wings - if they will eat the bone it will help the diarrhea.
Do this for a few days and they should be better.
Then, I suggest you stop feeding that horrible cat food and feed them something better. As you said there's not much choice in China, so go for a raw diet. That's the best diet they could have anyway, so you're in luck!
The raw diet is simple:
Raw meat (including hearts) should be 85% of the meal. Raw organs (liver, kidneys, lungs, etc) should be 5-10% of the meal. Raw bones (choose the smallest bones you can find, the tip of chicken wings or the chicken rib bones/breast bone) should be 5-10% of the meal.
It's easier to get the proportions right if you make a big batch and freeze it in portion sizes
Make sure the meat is made for human consumption.