The former Take That star knows the life of a boy band all too well and revealed the 'Little Things' hitmakers are like family to him.
He told the Mail on Sunday newspaper's Event magazine: ''They are [my brothers]. I sang 'She's The One' with them on 'The X Factor', so I've seen various stages of their growth.
''And I've been a member of One Direction, as it were. So I like seeing how they're doing.''
The reformed bad-boy - who is married to Ayda Field, the mother of his daughter Theodora, nine months - revealed he is ''envious'' of Harry Styles' womanizing, partying lifestyle.
He added: ''[I've been] following Harry and his messy romances and being slightly envious.
''Checking out their new tattoos and me showing them my new tattoos. I'm in the One Direction bad tattoo club. I have bad tattoos - so do they.''
Robbie also told of how he sees a lot of himself in the group, although he said the quintet lacks a calming influence like Take That had.
''It would seem that it's a band of Robbies and [Mark Ownens]. It's going to be fascinating to see what happens. You know, their journey.''