Re: Siblings and size difference
She absolutely shouldn't have been sold at that size. The average 6 week old is around 3x that weight. I would suggest getting a digital scale that measures grams, if the one you're using doesn't have that capability, and you should weigh her daily to monitor how she's doing. She may never be a normal size but she could do well at a small size. The main concern is whether she's gaining weight, even if it's just a very slow gain. She might be too small to chew kibble yet, even a small kitten kind, so it could help to crush it into very small pieces - almost a powder. You might want to supplement her with some kind of soft food like a meat baby food.
I'm going to PM you the email of a friend of mine - she's a breeder and is working with a very undersized baby (9-10 weeks old now). The baby wasn't fed properly by its mother because she wasn't producing enough milk. I don't know the exact size now, but the baby is slowly gaining weight. I'm sure she'd be happy to give you more specific advice and tell you what's worked for that baby.
USDA licensed breeder in Port Orchard, WA.
Volcano View Hedgehogs