If she starts shaking it could be a diabetic fit, unfortunately hamsters are prone to this. Look up the symtoms. If it is you can try putting a small amount of maple syrup on your finger for her to lick when she starts shaking.
You need to get her drinking. Really urgently. You could try putting a very small amount of sugar in the water to make it taste nice. Not sure if this is a good idea if it is diabetes though.
Also give her a teaspoon of yogurt, as it will get some type of liquid into her system. It's fine for hamsters to eat. Just make sure it has no sweeteners or artificial anything in it. Get organic if you can (its only about £1.40 for a HUGE tub and you only need a teaspoon) or baby yogurts as they are less likely to have additives. Obviously dont giev her chocolate flavour or toffee flavour or anything else artificial.
Another trick to rehydrate them is to soak a small bit of apple in water before you give it to her, and make sure she eats it. Cucumber is very high in water as well, like 80%
I'm not sure, but Im pretty sure you arent meant to feed them watermelon as it can give them diorrhea. Google it?
Try with your parents because it seems like shes suffering. Could you do chores or use your pocket money for the vets?
Good luck and dont feel too bad, all you can do is your best
and good luck!