The 25-year-old singer - who is set to tie the knot with Sam Cooper in Gloucestershire, South West England, on June 11 - has reportedly given British TV presenter Miquita Oliver the honour at her nuptials, while her younger half-sister Teddy Rose is to be a flower girl.
An insider told The Sun newspaper: "Lily and Miquita have been friends since childhood so there was always going to be a good chance she would be the maid of honour.
"Miquita knows Lily inside out and is bound to put on a great party for the hen do. They`ve grown up a bit now but both of them still know how to hit it hard."
Meanwhile, the `Fear` hitmaker has allegedly snubbed supermodel Kate Moss by not including her in the 300-person strong guest list for the wedding after she heard the 37-year-old beauty was planning on holding her hen party on the same night as Lily.
A friend of the star told the Mail on Sunday newspaper: "Lily and Kate are more like `frenemies` these days. Lily was upset when she heard that Kate was throwing her own hen night on the same weekend as hers.
"Lily then brought hers forward to May, but the whole thing has left a bitter taste. Kate hasn`t received a proper invitation to the wedding and she won`t be sending Lily one to hers in July, either. It`s awkward as they have mutual friends who may feel divided."
However, Sir Elton John and his partner David Furnish will be attending the nuptials, provided they can find a babysitter for their four-month-old son Zachary.
The source added: "Lily hopes Elton and David Furnish can come if they get someone to look after their son Zachary."