As it was explained to heal her sister, nothing comes without a price, and the price to heal her sister most likely would have been her life in her original world so if you think like this if she can't live in her world just let her live in another one.. I'm not really sure about this explanation as I said its been a long time since I read the novel but it was something in those lines
There is an English translation but I think either they dropped it or they are slow, here is the link:
https://www.novelupdates.com/series/miniature-garden-chemister/
I read it mtl'd from the raws here is the link if you wanna read it in mtl:
https://ncode.syosetu.com/n4084cc/
Spoiler:
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The ml is a god, she becomes a goddess or has the power of one and she can stay with him forever now, he even let's her visit her family at the end of the story (he couldn't before because he can't interfere with humans much, and since she became a goddess? He can let her visit her old world/realm and she lives in his/ the gods realm now, different from the world that she is living in right now)
He (the god) also had already informed her family that to heal her sister she went to another world and that she is living well and is happy, he also talks with her sister who always sends to her clothes that she suddenly finds like the pajamas, after a while the sisters can send letters to each other too.
About ikuru, she breaks the curse and he goes on to an adventure to become as strong as his master.
Its been a while since I read the web novel so there may be some things different especially about the goddess part I can't remember it well.