AMRITSAR, India — For seven decades, Sudarshana Rani has ached to learn her younger brother’s fate. She was just a child when the communal bloodletting that surrounded Britain’s 1947 partition of India wiped out nearly her entire extended family. But in the paddy fields that became execution grounds, there was one body she did not find: that of her 5-year-old brother, Mulk Raj.

Ms. Rani, a Hindu, and an older brother were sheltered by a Muslim classmate’s family before they abandoned their home near Lahore, which became part of the new Muslim nation of Pakistan. In India, they built anew. The brother, Piara Lal Duggal, retired as a senior officer in India’s state bank. Ms. Rani raised children who are now doctors and bankers.

Yet her mind remained with the brother left behind. Had Mulk Raj made a run for it and survived? She has imagined him searching for her; she saw him everywhere and in everything. Even a family movie outing a few years ago became part of her long, quiet search.

“I thought maybe this is my brother — they made the film about him,” she said about the 2013 biopic of Milkha Singh, the star sprinter who had overcome his own family’s massacre during partition. “I walked around the field, I saw everyone — not him,” she said of that long-ago day in the rice paddies. “Maybe he told his story.”

The chaos, confusion and religious violence that accompanied the cleaving of Pakistan from India 75 years ago this week resulted in the deaths of up to two million people and unleashed one of history’s largest displacements, with Hindus and Muslims from once-mixed communities rushing in opposite directions to new homelands created along religious lines.

In the decades since, the divisions have become more rigid than ever, the frontiers fenced and heavily guarded, after repeated wars, cross-border terrorist attacks and the backlash of swelling nationalism. To this day, despite a vast shared heritage, the two countries remain estranged, their guns fixed on each other and diplomatic ties all but nonexistent.