Warning: contains SPOILERS for Bridgerton season 2.
The terms Kate (Simone Ashley) and Edwina Sharma (Charithra Chandran) refer to each other in Bridgerton are "Bon" and "Didi," respectively, and the names have an endearing significance. In an effort to bolster diversity in the show adapted from Julia Quinn's bestselling romance novel series, the Sheffields become the Sharmas. Likewise, Kate's full name changes to Kathani, which was originally Katherine in the books. The Sheffield name remains in the show through Kate and Edwina's mother, Lady Mary Sheffield Sharma (Shelley Conn). She married below her class to a clerk from Bombay - presently Mumbai, India - and immigrated there to be with him and his daughter Kate, causing a scandal in her family.
Bridgerton season 2 centers around Anthony Bridgerton's quest to find a wife. As Bridgerton's Mr. Darcy equivalent, Anthony carries high standards for the bride that's to become his viscountess, vowing to only marry the season's diamond. Kate travels to London to support Edwina's courtship and becomes exceedingly protective of her when Queen Charlotte (Golda Rosheuvel) names Edwina the diamond, in turn becoming the most courted-after lady in the ton. Through Kate and Edwina, Bridgerton season 2 focuses on themes of family relationships and personal growth. Kate wants to bring honor to a family from whom she feels the need to earn respect, and Edwina reflects on her sense of duty concerning her sense of self and personal desires.
Despite the tensions that arise from their journeys of self-discovery, Kate and Edwina strongly care about each other. Just as Bridgerton season 2's Kate and Anthony love arc is perhaps stronger than Daphne (Phoebe Dynevor) and Simon's (Regé-Jean Page) season 1 relationship, the familial love between Kate and Edwina is more effectively presented with the help of their "Bon" and "Didi" titles. Kate calls Edwina "Bon," which means "sister" in Bangla, also known amongst English speakers as Bengali. "Didi," as Edwina calls Kate, means "elder sister" in Hindi. Essentially, the names are meant to serve as terms of endearment.
As noted in the two different tongues the sisters use to refer to each other, India has a complex language history. The country has 22 official languages today, giving India the highest amount of official languages in the world. Bangla and Hindi are among the country's official languages, yet Hindi is by far more prolific in Mumbai than Bangla, which is spoken primarily in northern and central India. In fact, while Kate's real name Kathani is technically Tamil, the word "kathani" is also Hindi for "statement" and could have been a play of words on Bridgerton's part to reflect Kate's outspoken nature.
This isn't to say that the languages of their terms of endearment reflect fluency in both of those languages. While they're surely fluent in Hindi, their familiarity with Bangla is more likely the result of living in a cultural microcosm like Mumbai, where they'd be exposed to dozens, if not hundreds, of languages. This isn't even considering the fact that the Sharma ladies are also highly educated. Still, criticism on social media and discussion forums highlighted that by using Bangla and Hindi without committing the characters to a particular ethnicity, the show conglomerates Indian culture as one entity and creates misrepresentation. As well-intentioned as Bridgerton may be, this wouldn't be the show's first critique of this account.