Save mystique

Mystique is a ten week old kitten she is light in the room. she is sick she has fluid in her belly and a little internal bleeding. The vet wants my family to put her down.  We can't just do that without knowing what it is that is causing her this much pain and if it is fixable. Could you please help us save our kitten she is a huge part of my family and without her she will be greatly missed..we don't want to lose our pet!