iTunes Match doesn't download anything by default. Once you have your big library uploaded or matched (or both) and the process has finished in your desktop computer, all you need to do is use the same iTunes Match account on your laptop. It will automatically bring all your tracks, playlists and such, but they will be stored in the iCloud servers. You can listen and buy music and such, but they will only be temporarily downloaded/streamed so you can listen them. If there's a particular song/album that you want to have locally stored (in case you don't have connectivity), you can click the "download" icon next to each uploaded/matched song and iTunes will automatically start a download process.
If you later want to remove the downloaded track(s), you can simply remove and move to trash. There will be a checkbox asking if you also want to remove it from iCloud (and you shouldn't check that unless you want to wipe the track!).
So as you can see, you can download your songs, remove them, re-download them as many times as you wish (I think). But if space is your concern, you don't need to download them in order to enjoy your music. I have a Mac Pro with a 90GB library that has been "matched/uploaded" and tho I keep the library in the Mac Pro (for backup purposes), my Macbook Air with a 250SSD doesn't have a local library, I just listen from the Cloud.
Bear in mind that when you press play on a non-downloaded track, there's a slight 1-5 sec delay before it starts. If you have a good internet connection, the track is downloaded in 2-5 secs too, so this is not a huge problem. If you're playing an Album from the cloud, iTunes starts downloading the next track before the current ends, so there's no unwanted gap. It comes to my understanding (but I haven't taken a look yet) that these temporarily downloaded tracks eventually go away, but not instantly, for if you replay the same track a few minutes later, it's still there.
I've come to like Match, it's exactly what I was looking for. Having the ability to "carry" my 90GB library when I'm traveling (and where I need Internet connection anyway) without sacrificing 90GB of an already small 256GB drive.
I am even considering removing some tracks from my Mac Pro (tracks that I don't really listen too much or I don't really care).
Happy Matching!
In general it's recommended to have one computer account per user (in your case one for your sister and one for yourself), each with its own settings, home folder, iTunes library etc. This also takes care of separating mail, calendars, iTunes Match settings etc. without having to go through a lot of loops.
In addition you need a separate AppleID for each user (especially if both want to use iTunes Match independently).
Best Answer
In Settings -> Store, make sure you’re signed into the account that has all the music/media/etc. (iOS lets you have one Apple ID signed in for iCloud and another for store purchases).
Then, in Settings -> Music, turn iTunes Match on. In iTunes for Mac, go to Store -> Turn Off iTunes Match.
The iTunes Match will then only be on your iTunes devices, but your Macs won’t have any iTunes Match.