My cat does the same, I adopted his mum who was destitute hungry and cold, took her to the vet, got her vaccinated etc and after a few months of good food realised she was pregnant! I nearly had a heart attack when she gave birth to 2 kids, now I had 3, the vet thought she'd been neutered but...
I wouldn't experiment with any human food like sugar, being a baby shes delicate but I think she needs a foster mum - I'd try a cat shelter adopt a mother and see how you feel adter few weeks. Its very nice of you to adopt her