I swear to you Audrey, if I went to a breeder's home that was raising a bird for me that I was told was 6 weeks old, then arrived and learned it was 3 weeks old, and that the sibling to this bird was half the size of the bird I was intending to purchase, I would NOT have taken home the smaller, weaker baby. I would not have taken the older baby either. How could I imagine I would do a better job of getting this baby to maturity than the breeder? Why would I do business with a breeder who would allow me to take either of these 3 week old chicks???
The OP did not meet the breeder at the side of the road and have the bird slipped to her in a box, sight unseen. It was a choice to leave that home with this baby, who would have stayed with an experienced breeder/handfeeder who was responsible for this baby and who should have KEPT the baby for many more MONTHS.
I probably would have questioned the breeder up one side and down the other on what was the prognosis for the smaller clutchmate, in particular wanting to know why a baby that was only 2 days younger than it's clutchmate was half the size, and would have asked if the vet had been contacted. But would I have walked out of a breeder's HOUSE who was breeding birds to sell with an undersize unweaned chick? NO!!!!!
Of course I want to see this baby recover. I think it is very important for this thread to contain the same warnings we always give:
DON'T BUY / ACCEPT UNWEANED BABY PARROTS