The pair, aged 18 and 24, were beaten to the ground, repeatedly kicked and left lying on the pavement on Sheaveshill Parade in Barnet.
They both went to hospital the following day, with the older brother suffering a broken jaw.
Police said they were approached by a group of men at around 11pm on July 2.
Detectives are appealing for witnesses and information following the short attack.
DC Colin Chandler said: “This was a violent attack on two brothers which left them lying on the pavement. The callousness of those involved is truly shocking.
“The area where this attack took place would have been busy at that time of a Friday night. I would ask anyone with information to come forward and speak with us.”
Despite extensive enquiries, including a search of local CCTV footage, officers have been unable to identify any suspects in the investigation.
There have been no arrests and enquiries continue.