PLEEZE relocate another chick to grow with this little one!
Two chicks can grow up quite well in a rubbermaid container that is kept warm. No mother is NOT a problem.
But what is a problem -- what is absolutely cruel, IMHO -- is to make that little one live a life of isolation. What will happen when she is to be merged with the other chicks later, when they are bigger? If she is alone, with no fellow chick to merge with her, she will be cruelly pecked and picked on by all of the others, with nobody to help her or stand up with her.
I have successfully merged two recently with a larger group (my broody only hatched three, and the roo had to be rehomed). But they clung together for dear life those first few days. They really needed each other during the merging process. I cannot even begin to think how horrible it would have been, if they had not had each other during that difficult time.
Please, bring another chick to live with this little one.
OH, and one other suggestion -- you might put a mirror in there with the two chicks. I did that with the most recent two that my broody hatched (I had to take them away from her because she had diarrhea), and they are growing quite well in their rubbermaid tub with their little mirror. They are now six and a half weeks old and are still living inside that rubbermaid container because of the really cold weather outside, and it's working out. (The broody returned to her flock over a month ago).