As Marisa began to play a few notes of Mozart's Violin Concerto No. 3, she remembered how much awe she used to feel for her little sister as she'd fly through new pieces as if she'd written them. Even before they began to play violin, Sam had proven herself to be gifted with music. She sang before she could even talk. The two girls had shared a room, and Sam would sing herself to sleep every night—“Me, me, me, me . . .” she'd sing, always right on pitch, right in tune.

Marisa would sing harmony. She couldn't wait for Sam to learn to talk—and as soon as she did, the girls sang rounds, played musical games, wrote songs for their parents and each other. Sam sang like an angel, and she played like one too. At the age of four, she could pick out songs on the piano. Marisa taught her everything she knew, and before long, Sam was teaching her back.

The two sisters would sit on the back porch, practicing songs in the light of the moon, and sometimes it seemed they could continue all night, just to be together. Marisa remembered how their bows would move in unison, playing the tunes, and how she would feel so lucky to have a sister who loved doing the same thing.

“We can do this together for our whole lives,” Marisa had said when she was nine, still called Patty.

“What do you mean?” Sam had asked.

“No one else will know all the same songs . . . we'll play these forever, the two of us, until we're old.”

“What if we move apart from each other?”

“We won't.”

“But sometimes sisters do,” Sam pressed.

“Well then,” she said, “we have to make a pact. If we do ever live apart, we'll always come back together every summer and play.”

“Do you promise?” Sam asked.

“I do,” she said. And they shook on it, right there under the rising moon.

For a long time, the promise never came up. They were so close, there wasn't a summer—or any other season—that they were apart. Marisa's first year of nursing school was the hardest, because Sam was still a senior in high school. But as soon as she could, Sam enrolled in Johns Hopkins just like her older sister.

Tuition was expensive, so at the end of every school week they would pack up their equipment and go to Irish bars, to play music and earn money to pay for their education. Their shows took them to places with names like Molly Maguire Pub, Blarney Stone, Galway Bay, Moran's Ale House. The crowd would be full of young people with bright eyes and open hearts, who would sing and stomp to the music, who would toast the sisters.