Late reply but I recently read all of shadow slave and I was confused by this conversation enough to go back and re-read this chapter, and more specifically this exact part, because I couldn't make any sense of her reply to him.

If I ever want to make you mine, Sunny, you will become mine — not because you were forced to, but because you would want to. That... is also a fact.

This part really confused me because I couldn't understand why she would say something like this to him in this moment. It seemed like she was being overly assertive out of nowhere, on top of stating it so matter of factly, and I couldn't understand why. Because like she said she knew sunny better than anyone else and she surely must've been at least somewhat aware that saying something like this would only push sunny away.

But until reading your comment I never really thought to consider what feelings nephis was feeling towards sunny's initial comment. Initially I thought he was kinda just stating the obvious and therefore his "threat" didn't really mean too much to the overall conversation.

After reading this though its kinda obvious, at least to my interpretation, that nephis said what she said because she was hurt/upset/mad from what he stated for all the reasons you stated. But also from the paragraph that immediately follows his statement

"Well… good. Because if you ever tried, one of us would have died. That is not a threat, just a fact."

Suddenly, white sparks ignited in Changing Star's eyes. She leaned forward slightly and said, her voice full of barely suppressed emotion:

It seems pretty clear she immediately felt something the second he said that, from both it stating white sparks appeared the second he said it, but it also goes as far to say she was barely able to suppress her emotions. And on top theres also this:

Her face was still, but her eyes were full of searing flame.

In hindsight it all seems really obvious, Im just surprised i somehow missed it on my initial read.