Dear Martin by Nic Stone

When Suzette comes home to Los Angeles from her boarding school in New England, she's isn't sure if she'll ever want to go back. L.A. is where her friends and family are (as well as her crush, Emil). And her stepbrother, Lionel, who has been diagnosed with bipolar disorder, needs her emotional support.
 
But as she settles into her old life, Suzette finds herself falling for someone new...the same girl her brother is in love with. When Lionel's disorder spirals out of control, Suzette is forced to confront her past mistakes and find a way to help her brother before he hurts himself--or worse.