Oof, some twists I was not expecting
As far as rejected mate stories go, this one was pretty good. The plot was excellent, it's just that the execution wasn't as good as it could have been. "Rosie" is the daughter of the pack's beta. When she is 16, the Alpha's son, "Reese" realizes that she is his mate on his 18th birthday. There is a pretty big betrayal that has several layers that I was not expecting. Actually, there are multiple betrayals that will have you thinking "whoa . . . wtf did I just read?!"
There is a secondary story percolating around second chance mates that is also pretty good, but the main rejected mate story is definitely front and center. I do recommend the book on the merits of the plot.