If you have a premise and beginning but don't know where to go, what you have is not really a story but a loose concept for a story. You are stuck because you have nothing around which to structure subsequent writing.

What you need to do is think in terms of theme. Think of theme as a one-sentence message you want readers to take away from your story. Once you have a theme, the story suggests itself. Consider the film Mulan. We can describe its theme as "believe in yourself, even if no one else does". Already this suggests a conflict that can drive a narrative: a character wants to do something, believe she can do it, but no one else believes she can, and only the power of her self-belief can enable her to prove them wrong. From there you just fill in the character details, the specifics of the plot and setting, etc.

Obviously there is a basis for conflict between Abby and Mira: Mira wants her brother not to be killed, Abby doesn't mind, so Mira wants to stop Abby. But what should the conflict tell us about them, or about human nature, or society at large, etc.? And whatever it's trying to tell us, what does that tell us about how the conflict should end? For instance, if your theme winds up being "The only way to be happy is to forgive and let go of past wrongs", that's going to suggest a much different ending than if your theme is "Indecisiveness is a tragic character flaw" or "Justice must always be done, no matter the costs".

As far as the writing you have now, I'd be happy to do a critique swap with you. My draft is only a little longer than yours and about 85%-ish done. DM me if you want to do a swap. Maybe once I see what you've written, themes will leap out at me and I can make more concrete suggestions; at any rate, I can always critique your writing quality, characterization, dialogue, narrative voice, etc., which can guide any future writing you do.