Two and a half stars, rounded up.
--The summary is misleading. The secret has nothing to do with Mary or their mother; although why Mary ran away/was hiding (which the prologue tells us) is revealed at the end, it's not the main secret.
--For a character-driven book, there didn't seem to be a whole lot of personality in the characters. I was mostly able to tell them apart from their circumstances--Martha's alcoholism and recent divorce and music struggles, Pfeiffer getting fired, Hadley's rocky marriage to a congressman. Beyond that, they don't have particularly distinguishable voices. Not that mattered so much in the narrative, it was still fine, it just wasn't so great for something that is so heavily character driven.
--Each of these characters has a reason to be angry, upset, hurt, emotionally volatile, or anything. That's the biggest reasons these characters fail to draw you in: they don't ever have a real reaction to anything. There's no stress, no drama, no misery, no distress. Everyone's doing just fine during these emotionally unsettling times. Their great-aunt's funeral? No tears, no real regret for leaving and never visiting or writing. A recent stint in rehab and a divorce? Hell, we don't even know why they divorced. An impending divorce and a marriage based entirely on business? Again no regret for time lost, and no sadness over what you might be losing. Or even relief. Lost your job and can't get another one? Pfeiffer has the most emotion out of all of them, but even then, it's entirely too placid. That's the main issue here: everything is placid. Dramatic circumstances, detached serenity in handling said circumstances, thus no impact.
--The romances are... there. Hadley and Brody have a history, but even then, their relationship isn't really built up, but supposedly they've never stopped loving each other. Pfeiffer and Luke aren't remotely built up either, and since they just met this is especially egregious. Neither of these romances have any foundation, so why we're supposed to root for them is a mystery to me. They're just shoved in there for a happy ending.
--Rushed ending with little build up; this "secret" is guessable from the introduction of the mystery at the halfway mark. Only one semi-twist, and again, no emotional reaction to make it affect you in any way.