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Nehru's daughter married Feroze Gandhi.So she was called as Indira Gandhi and the surname "Gandhi" continued.
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Indira Priyadarshini was the Daughter of Pt. Jawaharlal Nehru.When she married  Feroz Gandhi, her surname changed to Indira Gandhi.Sonia Gandhi is the Daughter-in-law of Indira gandhi and spouse of Rajiv Gandhi and hence the surname.Rahul Gandhi is  the son of  Sonia Gandhi and hence the Surname "Gandhi"