Sorry it took so long for me to read! LOL
The Stormchasers by Jenna Blum is about Karena and her twin brother Charles, mostly. Charles has bipolar disorder and Karena wants to find him when he disappears. They haven't talked or seen each other in 20 years. Karena joins a Stormchasing group in hopes of finding him, for Charles loves to chase storms. Along the way, she falls in love with Kevin, a tourguide. A couple days after they find Charles, he goes manic and tells Kevin about the man they killed while chasing a storm when they turned 18. Kevin gets mad and Charles (when he goes back to himself) wants to tell the Sherriff. To find out what else happens, read the book. I'd reccomend it for 17 year olds and older, but be warned, some parts are extremely boring!