Sometimes its hard to let it go.

Thats what Idina Menzels younger sister learned while growing up in the shadow of the future Frozen star the Anna to her Elsa.

As Cara Mentzel writes in Voice Lessons: A Sisters Story (St. Martins Press, out now), the three-years-older sibling she calls Dee (or Dina) was three years better than me at everything: tennis, painting, calligraphy.

Idina was light-years superior at singing. She was better than anyone we knew, Cara writes.

Despite Caras own interest in the performing arts, she couldnt help but feel constantly upstaged. Wasnt the risk of failure greater for me than it was for [my classmates]? she writes of her anxiety during a ninth-grade chorus audition. If I sang, I wasnt going to be compared to Nicole or Natalie, I was going to be compared to the best to Dina. It meant that if I failed, my failure was bigger than anyone elses. It meant that my mistakes would be louder.

When I was younger, I spent so much time figuring out how I wasnt like her.

Over the years, the sisters lives diverged. Idina went on to Rent, Wicked and a 2003 marriage to heartthrob actor Taye Diggs. Cara, meanwhile, moved to Colorado, became pregnant at 23, wed and had two children by 28 before her marriage imploded.

A year or so after the split, she slapped her ex and was arrested for domestic violence. To a good girl from Woodbury, [LI,] she writes, nothing says rock bottom like a mug shot.

Her sister bailed her out, then helped her buy a house and get her life back on track.

For Mentzel, now 43, Voice Lessons is more than a memoir its a study in deep gratitude and envy.

When I was younger, I spent so much time figuring out how I wasnt like her, Mentzel, who uses the familys original surname spelling, tells The Post. As I moved away and [matured], I was able to think about who I am, as opposed to who Im compared to.

Nevertheless, her sisters shadow was difficult to escape even in moments of self-reflection.

In the book, Mentzel recalls an incident that took place shortly before the birth of her second child, when her Broadway-star sister asked if the expectant mom dreamed of something more for herself.

The question was difficult to ponder. Did I have a baby because I needed attention, because she had Rent and recognition? Cara writes. I desperately wanted the answer to be no because if it wasnt, then in some obscure way, Dee was the impetus for the best thing Id ever done that is, start a family.

Eventually, Mentzel finds her own song, returning to school and earning a teaching degree. Now remarried and working as a freelance writer when shes not crafting lesson plans, she says her biggest source of sisterly stress these days is finding the perfect wedding gift: Dee, divorced from Diggs in 2014, married actor Aaron Lohr in September.

The sisters even sing together well, sometimes. For the audiobook version of Voice Lessons, Menzel persuaded her author sibling to record a cute duet.

Just dont get too excited about a live rendition.

My editor was like: Would you perform it on talk shows? Mentzel says. And I was like: Did you read the f king book? No!

Courtesy of the Mentzel Family

Courtesy of the Mentzel Family

Courtesy of the Mentzel Family

Courtesy of the Mentzel Family

Courtesy of the Mentzel Family

Courtesy of the Mentzel Family

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