Kittens are often free to good homes although some cat rescue places will ask for a small donation to help cover their costs. Cat breeders, who are in business, will charge for pedigree kittens but to me *pedigree* = inbred and may lead to inherited disorders.
As a kitten may well live for some 15 -20 years, you really should ask your sister if she is willing to take on this responsibility, plus it really should be her who chooses the kitten - they will have to live together after all.
May I suggest that if your sister does want another cat, she goes to a local cat rescue place and has a look at both older cats as well as kittens - with an older cat what you see is what you get, with a kitten you have no idea what their temperament is likely to be.