Kadambari Devi was Rabindranath’s sister-in-law (his older brother Jyotirindranath’s wife) and his muse. Kadambari (originally named Matangini) was born in 1859. She was nine when she entered the Tagore household as Jyotirindranath’s bride-to-be. Rabindranath was seven at the time and admired her.
They became very close. His bouthakrun (sister-in-law) Kadambari often teased him but also asked him to read to her, while he “shared the benefits of my sister-in-law’s hand fan,” as he remembers. Kadambari Devi inspired Tagore in his poetry and gave him creative feedback on his literary work. The relationship between them has been controversially discussed in biographies and in the media.
When she was 19, on 5 July 1868, she married the thirteen years older Jyotirindranath. Her husband arranged for her education, as he believed in women’s emancipation. From spring to autumn 1883, Jyotirindranath, Kadambari and Rabindranath lived together in Karwar (south-west India). Kadambari Devi committed suicide on 19 April 1884.
The reasons for this are not clear, although all biographers emphasize that it was four months are Rabindranath was married. Rabindranath was extremely affected by Kadambari Devi’s death. He wrote many songs and poems in her memory, calling her his sweetheart and his queen. As he writes in a letter
“With her death it felt as though the earth had moved away from under my feet and the light had gone out from the sky. My world felt empty and my life dull. I never imagined I would ever get over the delusion of this void. But that tremendous pain set me free for the very first time. I realized gradually that life must be seen through the window of death in order to reach the truth.” 
Kakar summarizes the role of Kadambari for Tagore:
“Some of Rabindranath’s biographers have called Kadambari “the deepest female influence on Rabindranath’s youth”, “playmate and guardian angel”, “deepest heartfelt realisation in [Rabindranath’s] life”. But perhaps what she meant to Rabindranath is best expressed in his own words, even when Kadambari is not their explicit addressee: “The heart of human beings is like liquid, which changes shape if the containers are different. Very rarely it finds the ideal container where it will not feel the emptiness or constriction.” Kadambari was simply Rabi’s ideal container.” 
Dutta, Krishna, and Andrew Robinson. Rabindranath Tagore: The Myriad-Minded Man. New York: St. Martin’s Press, 1995, p. 63.
Das Gupta, Uma. My Life in My Words. New Delhi: Penguin Viking, 2006, p.97.
Kakar, Sudhir. Young Tagore: The Makings of a Genius. New Delhi: Penguin Viking, 2013.