One place to consider for young and fun is at IKEA. You can get a lot for your money. The furniture shares the same clean lines, so it's hard to make mistakes decorating. It's one stop shop. She can get her dishes, curtains, rugs, art... The best part is that if her taste changes within the decade or she moves into a house, she hasn't sunk a chunk of change into furniture she may outgrow.

It's amazing what people do with small spaces. The smallest exterior could look HUGE when you see the inside. Maximizing the space is always key when designing. Where to put what to get the best utilization. Always consider the flow of the space when decorating also! Make sure everywhere is easily accessible and easy to move around.

I would purchase with caution with IKEA only because they have a history of short-lived products. If your sister is renting I think IKEA is fine but if it is more permanent I would recommend a higher quality product with more reliability and durability. Good luck! :)