If your sister is not trying to autocross the car every day, and wants a tire with a still-smooth ride while retaining good handling characteristics, I wouldn't get her into a uni-directional high-performance tire.
You should seriously consider a touring tire. These tires are multidirectional, but still have a high degree of handling capability. They also handle rain and snow much better than a performance unidirectional. They can be speed rated up to Y (186MPH), and are often the tires found on high-performance German touring sedans. They also usually have tread life which will go to the 40,000 + mile range, which will likely be twice that of a high-performance tire.
I would recommend the Bridgestone Turanza LS. I recently replaced my worn Falken FZ451s with these tires, and am very pleased.
I am a hard driver, but I also use the car for commuting to work. I found the Falkens (they were high-performance z-rated unidirectionals) to have amazing dry grip and very admirable wet grip. However, the downsides were that they rode fairly harsh, were extremely noisy, and wore down very quickly.
I decided to try a touring tire, figuring the high-speed stability was still very imortant, the smoother ride and much quieter tire would make longer trips and my commute much nicer, and the extended tire life would be a nice perk too. I wanted to ensure I wasn't compromising handling too much, as I still like to drift here and there through sweepers, and will still throw the car into hard turns at high speed...so the tire had to be relatively performance-oriented. And I also wanted a better all-weather tire, since my high-performance summer tires weren't as happy in North Carolina last winter despite the ESP and 4motion keeping me on the road in the snow.
The Turanza LS was receiving excellent reviews on TireRack, and I had driven a Mercedes and a BMW with some of the competitors' touring tires and admired the smoothness and quietness while still delivering solid performance.
I have had the tires for a week and a half now, and the first immediate improvement is the complete elimination of impact harshness and bump-steer. Also, the noise is GONE; these tires are basically silent. The wet traction seems to be basically identical to the high-performance summer Falkens. High speed still feels completely stable and responsive. Handling overall is probably 92% of what it was...still excellent, very catchable, and the tires do not protest when pushed. I notice a slightly lower limit before traction loss, but these limits are far beyond your typical driver (high speed through an arching exit ramp caused the car to drift very controllably through the entire length of the ramp, whereas before it would have stepped the rear out a few times while otherwise sticking).
Overall, the tires are barely a notch below the capabilities of the high performance tires, while adding smoothness, quietness, and all-weather tractability far beyond.
Check them out on tire rack for reviews and ratings...Bridgestone Turanza LS-Z. Highly recommended for anything less than a track car!