A Secret Love — Jean Thesman

Jocelin thought it was really super to be part of her best friend's family. But when Trudy's gorgeous, six-foot-four cousin Matt moved in, Jocelin couldn't decide whether to consider him a "relative" or a new love!
But there was something mysterious about Matt, and he used practical jokes to hide his true feelings. Jocelin knew it was going to take a lot of work to convince Matt that love was no laughing matter!