A story I've been thinking about

I won't be writing it for a while but it's called Bullet Witch and it takes place in a world that's half fantasy and half "modern" meaning they have magic but also have a few technological advancements. Born into this world is a child named Selena. She is born a witch and abandoned at infancy, taken in by two humans who raise her. Growing up with them, she doesn't seem to have developed any supernatural powers, this being the reason she'd been abandoned as you'll find later. But one night, her adopted sister is kidnapped and the rest of Selena's only family slaughtered. She turns to the gun that her adopted father had taught her to shoot and finds that this event has awakened in her some unique powers. While by no means an orinary spellcaster, Selena can channel her magic into the gun and create special bullets with various affects. Using this special skill among others, she sets out to locate and rescue her sister then take revenge on the ones who stole her family away from her.

That's as far as I'd gotten as far as plot is concerned. But that's really flimsy. The only elements I've established so far is this idea of "What do I really want to gain from this?" Like does she want vengeance, does she just want her sister back, what? I also need it to be more than just finding her sister, like a reason this happened in the first place. I have a very vague idea in my head but it couldn't hurt to hear some other suggestions.