I found it.

For a ton of good info on Petunia, check out the post – For members of the Order…/ Hogwart’s School of Witch…/ Great Hall, General Wiz…/ “Petunia Dursley? The mystery” started by “Miecha Potter” on 7/14/04

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In an interview, perhaps around 1999, Rowling was asked, “"I noticed in the Black Family tree that everyone is named after a constellation. Is this intentional? Does this have any bearing on the plot?" -- Answer -- "It's just one of those family traditions, although Narcissa breaks the trend. I had always thought of her as 'Narcissa' so I decided not to change her to match the others when I came up with their names. There's been a lot of speculation that she is in some way linked to Lily and Petunia, because of the flower theme, but I can put that rumour to rest here: she isn't related to them.”