Billy and Mako Vunipola have committed their futures to Saracens by signing four-year contracts with the Premiership champions.
The England internationals are two of Sarries' most established stars, with Mako arriving from Bristol in 2011 and Billy following two years later from Wasps.
Billy's new deal comes despite having been plagued by injury problems over recent seasons, severely limiting his involvement at club and international level.
"It's pleasing to get the opportunity to play here at a great club where I've spent most of my adult years," said Mako, who has made 144 appearances for the club.
"We're very lucky to have a family environment and we take it for granted sometimes. We're very lucky a lot of us have grown up together and genuinely consider each other family.
"You can see that on the field but off it we love each other's company; it's a bit cringe sometimes but for me that's what makes this club so strong."
Billy added: "I'm very happy to be staying here. It was a straightforward decision; it was always about how I enjoyed being around the club and the boys.
"I'm looking forward to growing as a group and growing friendships I already have. A lot of the boys who have been here for a long time have already re-signed like Brad [Barritt], Kruiso [George Kruis] and Duncan [Taylor] – the coaches too. I can't wait for the group to grow more and it’s something that excites me."
Director of rugby Mark McCall said: "Mako and Billy have both made outstanding contributions to Saracens over a number of years.
"They’ve played a very significant role in the development of the group both on and off the field and naturally we're thrilled they've committed their future to the club for another four years."
