That's one like a lot of people would donate to a school. It would be good for running Windows 98, and office applications, web browsing, email.
My parents still have an old Gateway with a 500Mhz pentium, 256 Ram (they upgraded that from 64) and a 20 gig hard drive. They use it everyday, for exactly those things. When I am there and I have to use it, I CAN"T STAND IT!!!! But they seem to have no problem, and just plug away on it. I keep on telling them to get something new and faster and they just stare at me and say "now that would just be just plain stupid to spend money on something if what we have works fine"
But what is it worth if they wanted to sell it? Nothing, absolutely nothing.
Same for the PC your sister has, it's not worth anything $$$ wise.