The duo are replacing agency founder Andy Fowler who is stepping aside to focus on major clients, including Sky.
Flack and Willmer will take responsibility for the day-to-day creative output of the agency across all clients, which include Sky, Sunny, Pearl & Dean, and uSwitch.
The duo joined the agency in 2009 and were promoted to creative directors in 2013. Their award-winning work includes the campaign to launch Sky Q (above), as well as Pearl & Dean’s "Tumbleweed", which won a bronze Lion at Cannes in Film this year.
Fowler, meanwhile, will focus on growing the agency’s biggest client relationships. He founded the agency in 2006 after leaving Sky, where he was creative director, and last year committed the agency to having at least one female director on every three-way director pitch.
Before Brothers and Sisters, Flack and Willmer joined Abbott Mead Vickers BBDO after meeting at college in 1999 and joined Karmarama in 2003, where they worked for brands including Ikea, Costa and Colman’s.
Fowler said: "Will and Aaron’s appointment as ECDs is fitting reward. They are hugely creative, inspiring and tremendous fun every single day.
"After eight years (or is it nine?) as Brothers, they’re ready for the challenge of building on our excellent creative reputation."