The backstairs revelations of the family life of Prime Minister Indira Gandhi paint a surprising picture of the cosmopolitan world leader acting at home as a traditional Hindu mother-in-law who turned against the widow of her youngest son soon after he died 18 months ago.

The story has taken the aura of a prime-time soap opera--the "Upstairs, Downstairs" of Indian politics--as Maneka Gandhi, the 25-year-old widowed daughter-in-law, described in detail what she called the mental "torture" of living in the prime minister's house after the death of her husband Sanjay.

"They have tried in every conceivable way to break me. But I have borne it all quietly for the sake of Sanjay and our son," said Maneka, a vivacious former magazine editor who is being pushed by some of her late husband's political supporters to take an active role in Gandhi's Congress-I Party.

As a result, Gandhi has cast Maneka from the family household. And Maneka, following tradition, has moved into a hotel with her 2-year-old son instead of returning to her own mother's home.

"After all," said Maneka in an interview with The Washington Post and The Associated Press, "I am her bahu daughter-in-law and still a Gandhi. She has every right to send me out of her house. After all, it is her house and she is my mother-in-law.

"I don't know why it has happened, and I don't think I have done anything wrong. I am ready to return if she Prime Minister Gandhi wants."

Maneka said she will look for a house for herself and her son Varun, who reportedly is the apple of his grandmother's eye.

All this shows how Gandhi, 64, managed a traditional Indian extended family at home--with her two sons, their wives and children all living with her--while projecting the international image of India as a modern state.

It is one more sign of the conflict within this country of 700 million between the ancient Hindu traditions and attempts to pull into the industrialized 20th century. It also shows how just below the surface of the most modern, western-educated Indian, such as the prime minister, one finds the traces of traditional Hinduism.

Gandhi's strong ties to the ancient codes of family life, set down in Manu, the book of Hindu social conduct, 16 centuries ago, seems all the more jarring since India is the world's largest democracy and one of three nations in the world ruled by a woman.

Traditionally in Indian families, a bride moves into the home of her husband's parents right after marriage, where she is considered part of the man's wealth. Except for occasional visits, she never returns to her family's house.

The bar against returning to her family home remains so strong that the young wife of a Calcutta executive, for example, must stay with distant relatives of her husband's when she visits New Delhi although her parents live there.

A bride's new home is ruled by the Indian mother-in-law, whose tyranny is so legendary that it forms the subject of many Hindi-language tear-jerker films. The new wife stands just slightly above the servants in the household pecking order, and she gains status only with the birth of a son.

In the Gandhi family household, Maneka related in the interview, her sister-in-law Sonia, an Italian woman married to eldest son Rajiv, was known as "bari bahu," or elder bride. Maneka, married to the youngest son, had less status--even in the right to express her opinions, she said.

But what little status Maneka had gained in the Gandhi household through the birth of a son ended with the death of her husband in an airplane crash in June 1980. While some of Sanjay's allies tried to push Maneka to the fore, the prime minister clearly preferred that her eldest son, Rajiv, take over as crown prince of the Nehru-Gandhi dynasty that has ruled India for most of its 34 years as an independent nation.

It was this conflict that led Gandhi, a widow herself, to throw Maneka from the house--a move that cast the prime minister somewhat in the role of a villain, to political analysts here say.

For in traditional society the widow remains the responsibility of her husband's family after his death--considered inauspicious but tolerated for the late husband's sake. She is relegated to an inferior position in the household.

Some Hindu sociologists believe this treatment has led many widows in the past to perform sati, the ancient custom of throwing themselves on their husband's funeral pyre to be burned alive. While this is outlawed in modern India, it sometimes still is performed.