I'll be completely honest with you. I'm a "Ford" guy, so I'm being honest here.
The media is going in overdrive on the Toyota recalls. None of what is happening is as serious as it is made out to be. The chances of your sisters Prius having the braking issue are very slim. There is a fix in the works for it, and it will be soon.
For now, keep the car. Toyota builds a wonderfully reliable, quality vehicle. I have no doubt that post all of this recall stuff, she will be very satisfied with it.
The reason you may feel like people don't want to take the car as a trade is because Toyota's are an unknown right now. Dealers don't know how much to pay for them because of the media madness that is going on, and how much they are actually worth.
I have faith in Toyota, even though I'm not a Toyota "guy." They have built fantastic vehicles for more than 60 years. This is just a lull.
Weather the storm, and your sister, I'm sure, will love her Prius.