Good luck with her. It's very hard to keep them as pets, but some manage it. They just weren't designed for long lives, unfortunately. At 4 1/2 months they have already well passed their recommended age for processing, which is usually 6 - 9 weeks depending on the size/weight desired. At 3...
How old are they? This is a Cornish cross meat bird? They are known for leg and heart problems and generally don't live long. They grow so fast and get so heavy that they often 'out grow' their legs. That she is so much smaller than her hatch mates indictates that she may have other issues...