I just found out that I had my sister's save file (we both share the Pokemon Moon cartridge) and I am wondering how to open another save file without deleting her's?
If you go through the process of installing CFW (custom firmware) onto your 3DS, it's possible to have two different save files via the use of a save manager. I am aware that this question is really old, but in case anyone else wants to do it, I'll leave this link here: 3ds.guide
(Copied my answer from a similar question. The same applies)
(I read that someone asked if this was legal. Yes, it is. As you are just modifying the hardware you bought. However, using PIRATED Games using Homebrew, is Illegal. Don't use Pirated games!)
Yes. This is possible, However for this you will need to install Homebrew. Homebrew is a Custom Firmware that allows stuff such as:
Custom Themes, Roms, Save Managing and Manipulating (Export a save, Create Pokemon on your PC using PKHeX, Then Import the save) (Or just export the save and it will be there) Etc.
I recently reset my Omega Ruby since I didn't know what to do in Sun, so I felt like replaying Omega Ruby. But Since I had a lot of Pokemon on there, I made a backup using JK's Save Manager. If you would like to install Homebrew, don't recklessly mess around in your 3DS's files, but instead use a video tutorial.
Here's some links I found to help with the Homebrew installation:
https://wiki.gbatemp.net/wiki/List_of_3DS_homebrew (List of Homebrew Apps that work with Homebrew)(How to use PKHeX and Save Manager with Sun and Moon. This is probably what you want. You can export the save and just leave it at that.) (How to install Homebrew on Any 3DS)
(I recommend you also save the files on your computer - in case something goes wrong (Rare chance), you won't lose your saves!)
If you don't want to Install Homebrew, then either buy a new cartridge, or delete the existing save.