One game on one computer, two origin accounts

Do you have sepearte Windows Accounts ? Would you allow her to login in with your account ?

I would say the easiest is to have two separate Windows Accounts which will give you each a different set of user files in documents. She should be able to just login with your account on her windows account and then have her own set of user files.

If you don't want to have different windows accounts, you could also just have the sims folder in documents each which you can switch in-between by renaming the folder to say thesims_you and thesims_yoursister. Who ever is playing just renames theirs to thesims.

If you don't want her to login with your Origin account, it should be possible to just buy it on hers as well and then use method above. You don't need to reinstall the game. I'm not 100% positive about this one but with the great game guarantuee you can give the extra copy back within 24 hours if it doesn't work out.