How can I get a visum with a longer validity than 1 month? Preferably 90 days?

Hello,

I want to travel China for 2,5 months. At this moment I am a student in Daegu, South Korea. I have an ARC which is valid less than 6 months at this moment. I seem to be able to get a visum for 1 month at several travel agency's. However no-one can tell me how i can get one for a longer period. Or I seem to have to Extend my visum in China? but how does that work?
I hope you can help me out :)
~Daan

And 2nd Question, is it also possible to get this visum for my sister from the Netherlands who does not have an ARC, just 30-day touris visum?