Yours is an original first generation i5, her's is the third generation i5 so her processor is newer and faster.
More importantly your computer is two years older then hers and since you do not know what temporary internet files and cache are then I am going to assume there has not been any system maintenece done on it so you have two more years of junk internet data, fragmented files, and extra programs loading up to slow your computer down.
Go here
http://www.piriform.com/ccleaner/download/standard and download ccleaner (download will start automatically).
When it firsts opens up, select "Run Cleaner" button and this will clean out old junk logs/history (this is all just saved up temporary system data, it is not going to mess with any of your files), you will probably be amazed at how much junk it finds.
Next go to the registry tab, run it, when asked do save the changes to somewhere on your computer.
Now go to tools, and startup, list in here all off the items in the list and I will tell you which ones you can dissable (i may need to know what printers you still use).
Lastly go to start and type "disk defragmenter", make sure C: drive is selected and then select defrag disk.
These steps are the core to doing a system tune-up, you can also go to Tools, Uninstall in the ccleaner program and uninstall software you do not use anymore.
No matter what your sister's laptop is going to perform a slight bit faster due to the difference in processor.