Leon and Alex originally entered the talent contest together, as a group named ITG (an acronym for 'In The Genetics').
They received mixed feedback at their room audition, but progressed to the second stage of Bootcamp - where Simon dropped a bombshell.
"Do you know what, I don't think you should be in a group," he said. "I think both of you should be competing on your own, because you're different singers."
The lads - who started out singing at a Yarmouth hotel alongside their mum and nan - agreed to split.
Both went through to the Six Chair Challenge as soloists in the Boys category, mentored by Louis Walsh.
First to perform was 22-year-old Leon, who arrived on stage in crutches after injuring his ankle while playing football.
The crowd got on their feet for his cover of The Chainsmokers' 'Closer', which included a rap he wrote himself.
Simon grinned: "I have to take credit for this, because it was my suggestion!
"Leon, you've come alive on your own. You're like a completely different person. The whole delivery, I thought you did amazing."
Louis agreed, adding: "This is the first time I've seen something in you."
Leon was rewarded with one of the five empty chairs - but unfortunately, things didn't go quite so well for Alex.
The 18-year-old's performance of Plan B's 'She Said' failed to win over Louis, who claimed his brother "has something more" than him.
The Irishman commented: "I think you should be very proud of yourself. You gave it everything, and I've seen such a big improvement in both of you.
"You were good, but Alex, there was no spark for me. For that reason, I'm going to say no. I honestly don't think this is going to work."
The audience booed the result and began chanting "bring him back", to no avail.
Simon at least offered Alex some parting words of encouragement, saying: "I think what you've done, giving your brother a chance means that you're a man. I respect you for that."
Before leaving the stage, Alex went over to hug Leon, who told him: "You've done so well. I'm proud of you, mate, I'm proud of you."
Although Leon got a seat, it's not yet clear whether he held on to it. Louis will reveal his final six Boys for Judges' Houses later tonight.