An Australorp and a Broiler! I'm curious where you got your chicks from... Hatchery? Breeder? Friend?
Unfortunately, Broilers are meant to do exactly what little Tess is doing.... grow FAST. She's not bred to free-range or really live beyond 8 weeks or so. They are commercial entities designed for the purpose of reaching slaughter weight as quickly as possible.
Eventually, her growth with exceed her body's capabilities to support her.
Honestly, I would feed her well, get another chick or two to keep Claire company, and prepare yourself to process your first chicken, or give her to someone who will appreciate a chicken dinner. If you try to keep her alive, she could end up suffering.
You would have to monitor her feed intake VERY carefully, but it's possible her organs would just start to shut down. They really aren't made to be "pets."
Forgive me if I'm coming off as cold or clinical. Broilers are what they are, and there's not much anyone can do to change that.
I hope Claire continues to thrive and that you can get her some more friends!
MrsB