What are upgradable parts on MacBook Air 13 inch mid 2013?

Well, I have good news and some bad news here ....

The bad news here is you can't upgrade this system that much. Your system has soldered down RAM which its not upgradable.

The SSD is removable, but presently there is no 3rd party options to upgrade it and you are currently limited to a very expensive Apple spares drive. While some original Apple drives can be found on eBay from a parts system I don't recommend going that direction as you could get stung if you're not careful. Besides they aren't very large.

OK now the good news!

Most of the time when we see a slow system it's a simple fix! You see your sister has file bloat! She has not left enough free space on SSD drive. So the first thing here is to clean house. Get rid of the unneeded junk thats built up, a tool like: Disk Doctor can help. You want to have 1/3 to 1/2 of the drive free.

The next thing here is adding some more needed storage as I'm sure your sister doesn't want to get rid of all her stuff she has, who does! She needs an external drive to hold things as well as be her backup drive (TimeMachine). The last thing here is to add some additional storage within her system using something like: Transcend Jet Drive Lite.

So, cleaning things up and adding an auxiliary drive as well as getting a larger external drive will buy here some time before she needs to look at upgrading here SSD drive internally. Hopefully, this year some options come online.