His bio is here, but the basics are: They grew up super close, since they were only 18 months apart. He had a rough time in school, and when he'd be sad about it, Lacey would come and kiss his cheek and say she loved him. When Nick was in 9th grade, their Aunt Carol (dad's sister) was in a car accident due to falling asleep at the wheel. Nicky made their dad promise he'd never drive tired, and he swore he wouldn't. He would pull off the side of the road and sleep if he needed to. A year later, dad stopped to rest on the side of the road and a semi hit the car. He was killed instantly, so the kids' lives were completely different after that. Nick had a weird experience in an English class with a meditative thing called "taste the lemon". When he was lead through it, and he 'tasted the lemon' he actually tasted it, and thought immediately of his sister. Later when they were on their way home and talking about their days, he realized that she'd been getting yelled at by a teacher when it happened. So his tasting lemon was tied to her. His psychic thing is Clairgustance & Clairolfactance (psychic taste and scent). If you wanted her to be psychic, she wouldn't have to have anything remotely like his.
Also, her name doesn't have to stay Lacey, of course. It's just been the placeholder.