Correct. The files would need to match the sister's personality (so no mommy voice, unfortunately), mainly be moans (and maybe a few clips of Onii-chan?), and be individual audio clips without background noise/ost that don't jump into the audio without warning or clip off at the end before finishing. So, the main parts of the adding such a thing would be finding said audio clips somewhere.
Most of these problems are linked to the loading of many txt files with the multi-lingual support the mod adds, so I don't know if the solutions we've found so far will help, but hopefully it does:
From the FAQ:
Q. The game is slow/has low fps!
A. A few people have found this issue starting with update 1.5.1, where I started to introduce a new system to the game. There are a few solutions currently presented by those who have faced this issue: 1) that the pc is struggling with the new loading times in game, and recommend either using an SSD to load faster or reverting back to mod update 1.5.0; 2) playing the game from within steam causes lag issues sometimes, so try using a pre-unpacked version from the game's main thread and see if your issue is solved; 3) running the game as an administrator.