Yeah, I know the translation isn't precise. But I do think it sounds better like this - 如果有妹妹就好了 instead something like 你只需要妹妹. Also, main reason cause I really loved the official LN/Anime so I was really inspired at the time when I wrote the story.
Your welcome.
It does sounds better than the exact translation
只需要妹妹就好了.
The original one...was quite the piece of a story xD.
Hope you continue to write this story and not drop it!