MAJOR SPOILERS
they aren't siblings, no no but in the past the ml drowned as a child and was saved by non other than our fml (who was a mermaid thus we always see snippets of her being compared to one). Won't give all the details so that you have something to look forward to but just a few more details is that in return for the fml to be human she had forgotten all about being a mermaid and the ml.
There is in fact a novel.
You can google the link for it.
To answer your question, yes she did choose to become human since the ml wasn't like that back then. They made a promise with each other. Her other sisters tried to warn her off of becoming human but alas. The young ml back then wasn't like the trash he was.

I wonder who she really is and what’s her identity.. like who are her parents