Question to pianists regarding a grand piano purchase?

Hi, I am considering purchasing a grand piano for my sister and since it is a considerably pricey investment, I just wanted to make sure I made the right decision. I am looking to buy something in the Yamaha, and Kawai range however I just saw some pianos from brands like Schaffer & Sons and they go for like $5,000 as opposed to brands like Steinway & Sons which require an investment of at least $25,000. I'm sure there is a difference in the quality of sounds what really is the difference between these two that creates a marginal difference of $20,000? I mean, my sister won't be an accomplished pianist however I would like to buy her a reliable good quality piano that she can trust for the rest of her life. Please excuse my ignorance and provide your input, looking forward to your responses, thanks!