I am still running OS 3.2 and would like to upgrade to the current OS, but I am not sure how to do this.
My iPad was set up on my Windows pc, and synced to my iTunes account on that computer. I no longer have access to that computer at this time (but I will, eventually...)

I am looking for suggestions on how I can update the OS without that pc. Can I bring it to the apple store, even though it was set up (formatted??) on a Windows pc?

The other option is that I know my sister has a Mac, and has never even set up an iTunes account on it. Hard to believe, I know! Can I use her Mac to update my Windows-ish iPad, and if so, what about all my data? I don't want her Mac to be my primary library, and I don't want all my apps to be erased. I am confused about all this and appreciate any advice. Thanks.

You can update with any computer without syncing or deleting data... But I would recommend having a backup.... You never know what might happen during an update.

The easiest thing to do would be to being it an Apple store... if you can get an appointment with a "genius". It doesn't matter what type of computer you use to sync your iPad with.