This appeared as "Sibling Rivalry" at AOL's MyDaily/HUFFPOST WOMEN. +++

"Whoever gets a D in math has to sit in back."

That's how I'd call shotgun when it was time to squeeze our teenage bodies into the family's tiny Honda.

My younger sister Monika didn't appeal to my mother at the wheel. This cruel, impromptu rule was imposed by a straight-A sibling, and grades were everything in our California household.

She'd trundle into the backseat, probably hating life.

Our oldest sister, whom we called the "Queen of Mean," was away at college but present in spirit.

Family films show that sister pinching me, a swaddled newborn. Then the 2-year-old beams when she notices the cameraman.

In another reel, her golden curls flying in the wind, she's pushing me -- a pigeon-toed toddler -- off the back of her moving tricycle.

We've got no babyhood footage of me doing that kind of thing to Monika, 15 months younger. Bad behavior would become apparent later, during years of being pitted against each other in academics, sports, music.

Maybe our parents encouraged the fractiousness as a parenting technique.

We three kids must have been more manageable as rivals and informers, rather than allies and colluders.

By school age, we were experts in our household's reward and punishment system based on my father's experience in the Army. Misbehavior led to grunt chores like scrubbing bathroom tile grout with a toothbrush.

Accomplishments won us R 'n' R passes to slumber parties.

We sisters provoked each other for extra privileges. "Look, Mom, I'm being good and she's not."

Eventually we'd harass each other for pure sport.

"That shirt makes your teeth look yellow," Monika would happen to notice as I headed out the door for a junior high dance.

The moment was a far cry from our brown-haired toddler years, when Monika and I adored each other. Back then, our favorite pastime was playing a game we called "French Lady."

A cosmopolitan fantasy of drinking tea from a porcelain set, old lady handbags swinging on our forearms, we were two preschoolers speaking in French accents. Continental ladies-of-leisure must have been quite a stretch in late 1960s Berkeley, a town known for hippies and beatniks.

In college, Monika and I struggled to rekindle our "French Lady" rapport. Tentative, well-meaning contact was always a hair's breadth from implosion.

Visiting her at school in San Diego, I offered her a new hairstyle. I wanted to try a razor technique I witnessed as a model for a European salon.

I was not a stylist, and the only tool she had was a Daisy razor. Monika decided to trust me anyway. Having girlie fun, we disregarded the probability of disaster.

I pulled out a back curl and scratched at it with the pink plastic razor. A few strands caught and held on the narrow blade.

I increased the pressure and suddenly the entire lock gave way. The razor plunged to her scalp.

"What was that?" Monika reached up to feel the new concavity.

"It's -- nothing," was all I could say, overcome by a sudden fit of giggles.

I wanted to keep going, to fix it. But she was already across the room rooting in the dresser for a hand mirror.

"I'm dropping you at the bus station right now," she screeched. "I want you out of my sight!"

A 500-mile bus ride back to my parents, a truncated vacation with my sister. No.

But what were my options? I didn't know anyone in San Diego.

Except my older sister. She hardly talked to me.

The wounded creature in front of me had invited me in. If I wanted to stay, I'd need to change the drift of the afternoon -- and the entire course of our sisterhood.

Imagining what my friends wanted to hear from siblings who tortured them, I started: "I didn't mean to ruin your hair. I care about you."

The words sounded so formal and undefended. Unlike me.

"I want you to be happy," I heard myself explaining.

She stopped screaming. This wasn't just about the bad hair cut.

"I love you, and I'm sorry," I finally squeezed out, a surprise sob in my throat.

To admit how much I felt for this brown-eyed girl in a Hawaiian shirt put me in a sad, vulnerable place.

We stood looking at each other from opposite sides of her cinder block dorm room. Tears started to roll down our cheeks.

Monika came in for a hug, whispering into my ear, "I love you too."

From that day on, we relied on each other as sounding-boards for shared anxieties and revelations. We began to appreciate our kinship -- and our kindredship.

Later, I was living 9,000 miles away when Monika needed surgery. She wanted me to take care of her. Arranging a medical power of attorney, she gave me the right to have her unplugged if something went awry.

That's when it hit me. My baby sister now trusted me enough to put her life in my hands. Beyond accident of birth, we chose each other.

The month I attended her pre-op appointments, shopped and cleaned for her was the best time we'd ever spent together.

She was a grounding family presence at my wedding, and soon I was able to return the favor.

Monika's house burned down. Returning to a charred pile of rubble, she wasn't able to function. I was her first call and we puzzled through the devastation with the same analytical skills I once used to banish her to the backseat.

I was thrilled when she claimed to friends, "I borrowed my sister's brain to start rebuilding my life."

Socializing in genteel situations is still one of our favorite things, dressing up and affecting our best sensibilities. Sometimes we don't bother to dress up, or drink anything. When we get together, what's important is that we bring our best selves.