I know what your sister is going through. I got a kitten a month ago because his mother had died. What I did was get dry kitten food and pour milk over it. The food get soft. I'd heat it up for a few seconds, and he went to town. He is now able to eat just dry food, but I still feed him the mush stuff twice a day. I leave dry food out for him to eat when ever he gets hungry. He has gone from an underfeed, almost dead kitty, to a healthy, fighting, hyper kitty.

P.S. The mush is something that my vet recomended for me to do.