Need advice on my sister's new cat.

A few months ago my sister's cat passed away and it left her devastated. She recently adopted another cat from a local shelter. She went in there and asked which cat had been there the longest and brought home a beautiful cat named Hazel. She has had the cat for two weeks and the cat is very standoffish and sleeps in her litter box most nights. Is there any advice I can give her to help coax the cat into feeling more comfortable or is it just a matter of time? Thank you for any advice.