Every Summer After question

Hi guys, I’m searching for a book I read some years back but I can’t remember the title or the author only the plot. A girl visits her father and his family in the summer and she starts having an affair with her half bother, father’s son in the new marriage. The family doesn’t find out about the romance until the 2nd book. In the first book, the boy has a girlfriend and she confronted them about it. Also, the boy and the girl were kissing in a tinted car. Does anyone have an idea what the title is? Thanks.