My cousin was put in a similar situation with her first cat. She was training in nursing and one of the requirements is to go on rounds with a midwife. One of the houses she went to had a cat with kittens, and as she had decided to get a cat she thought she'd give one of these little ones a home. (Part of the reason being she felt the animal care in the house wasn't up to scratch.)
She chose a kitten, then got a call when they were six weeks saying if she didn't collect that day the kitten would be giving to someone else. She knew they were too young but was also aware if she didn't take the little one there was no guarantee it would end up with someone who would love it as much as her. When she got to the house, the lady had given away the kitten she'd chosen anyway.....but she came home with a lovely black and white tom who is utterly spoilt and very much a part of her family.
She did have to give him milk substitute for a few weeks after she got him, on the advice of her vet so that might be something your sister should look into.
He's two now and is a lovely lad.