He is quoted by the Daily Mirror newspaper as saying: "I’ll show all the guests to their seats. And then I’m doing a reading. It’s her big day, so I hope she likes it."
Lily wrote a song about her actor brother, entitled ‘Alfie’, which contained the lyrics: "My little brother's in his bedroom smoking weed, I tell him he should get up cos it's nearly half past three."
The pop star’s 24-year-old sibling is not the only family member at Lily and Sam's nuptials with a job to do on the day.
The 'Not Fair' hitmaker has asked 12-year-old DJ Ted Lavender – the brother of her half-sister Sarah Owen – to hit the decks to the 150 guests at her wedding reception after hearing his demo tape, and he is "excited" about the prestigious gig.
He said: "I do a lot of practising in my bedroom and I don't get nervous, except at the beginning.
"But mostly it's just really exciting when the crowd gets going. Lily's really nice and I'm looking forward to really rocking her party."