Dandelions - Ash and Ruin Trilogy, #5

At fifteen years old, Gwen's world has ended. Not because she's gone through a break-up. Not because her parents are ruining her life with a lame curfew. Not because her grades are struggling. Not in the way a fifteen-year-old's life usually ends. At fifteen years old, Gwen has burned the dead bodies of both her parents and fortified her home against the plague-spreading monsters who killed them, waiting for her sister, Maggie, to make her way back home in the apocalyptic landscape that is the world now.
At fifteen years old, Gwen's world has ended, but she's not giving up. She's not giving up on life, her home, or her sister. Because all you have left when the world ends is hope, so that's what she's got, a BB gun and hope. Hopefully it'll be enough.