"There’s a really good young core in the team now with people like Jamie George, Owen Farrell, George Kruis, Jackson Wray, Maro Itoje and of course my brother," prop forward Mako told the club website.
"I’m excited to see how far we can go and I want to be around and enjoy my rugby with them and see how we can improve as players."
Mako Vunipola, 24, has been at Saracens since 2011 while number eight Billy, 22, arrived from Premiership rivals Wasps in 2013.
Both brothers look set to be part of Stuart Lancaster's England squad for the World Cup later this year.
(Reporting by Alan Baldwin, editing by Justin Palmer)
