The Warlock’s Kiss by Tiffany Roberts

The Sundering toppled civilization, killed millions, and filled the world with walking corpses and supernatural monsters. It took nearly everything from Adalynn—her dreams, her home, her parents, and her chance of beating her terminal cancer—but it didn’t take her little brother, Danny. She’s come to terms with her inescapable fate, and she’ll do everything she can to ensure her brother has a chance of survival in this new, unforgiving world.
When Danny and Adalynn discover a rundown mansion in the middle of nowhere, it seems like the perfect place for him to stay, but the mansion’s owner—a tall, mysterious, intense man who exudes a strangely thrilling energy—complicates matters. Though Adalynn doesn’t want to leave another person devastated when she’s gone, Merrick has other plans for her—and he makes it clear he will keep her, no matter what price he must pay to make it so.