Julia hadn’t been dating Jason long when she got pregnant. She wasn’t planning on settling down at 22, but when she met Jason’s sister Sienna, Julia knew she wanted to be a part of their family. Their parents had been killed in a car accident, so it was Jason and Sienna, but they looked out for each other. So Julia married him, and got a gift of a sister-in-law as well.

Now it’s many years later. Julia and Jason’s son is 15, a sweet kid turned surly teenager, and Julia and Sienna are business partners as well as best friends. But when Jason’s boss is killed, stabbed, suffocated, and his lips sewn shut, their community is stunned by the brutality of the murder. And then Jason ends up in the hospital, his car driven into a tree. He’s put into a medically induced coma until he can breathe on his own, and by then, the police are telling Julia and Sienna that Jason is a suspect in his boss’s death.

Neither Julia nor Sienna can believe that Jason would kill his boss, but Julia does know that he had been keeping secrets from her. She hadn’t been able to tell Sienna yet, but Jason had withdrawn a lot of money from their shared savings account, the one where they were saving for the second honeymoon in Europe, and when she had found out the money was missing, Jason said he’d lost it in a bad investment. Then she found out he’d lost out on a promotion he’d been hoping for. But murder? That was a completely different animal.

Sienna is certain her brother could never kill someone, so she does what she can to fight for her brother. Despite being told by their lawyer and by her ex-boyfriend, a police officer, not to insert herself into the investigation, she can’t stop herself from trying. But what she finds is an email from Jason’s coworker that might be about is infidelity and evidence of his boss’s possible financial crimes that is contaminated by her amateur detective work.

As Julia learns more of Jason’s secrets and starts to question if her husband is truly innocent, Sienna digs in deeper, a true believer in her brother’s goodness. But as the secrets come to light and the lies begin to unravel, will they free Jason or will the handcuffs come for him sooner because of what they find? And when everything is out in the open, will Julia and Sienna still consider themselves sisters, or will the weight of Jason’s crimes break them both?

Thicker Than Water is a taut thriller about a marriage and a friendship that were at one point unbreakable, but have been shaken and splintered by silence and lies. The dynamics between these two women as they dig for the truth is the heart of this story, their loyalty tested as everything they thought they knew turned upside down and inside out. Author Megan Collins does a deep dive into female soulmates with this story, and the results are stunning.

I really enjoyed Thicker Than Water. The relationships between Julia and Jason and Julia and her son are fraught, but it is her relationship with Sienna that takes center stage. Sienna’s relentlessness is powerful through Julia’s doubts, and Julia’s quiet strength is the perfect balance to Sienna’s heat. I loved how they could bring out the best in each other and learn from their mistakes to become better individuals. I was able to figure out fairly early on who the killer was, so the mystery is not the strongest part of the plot, but the story of the sisters-in-law make this a solid domestic thriller with lots of emotion and twists.

Egalleys for Thicker Than Water were provided by Atria Books through NetGalley, with many thanks.