The Fainga'a twins will reunite at the Brumbies with former Wallabies centre Anthony joining Saia for the 2017 Super Rugby season.
Currently playing with Kintetsu Liners in Japan, Anthony has signed a one-year deal to play in Canberra, where he grew up with his hooker brother.
The Brumbies have reinforced their midfield stocks, with inside centre Matt Toomua now with Leicester in England, utility back Robbie Coleman moving to the Western Force while five-eighth Christian Lealiifano's future remains in doubt as he battles leukaemia.
Brumbies coach Stephen Larkham said he was looking forward to having the experience of the former Queensland Red at the club.
"Having the twins returning will bolster our depth in key positions," Larkham told sanzarrugby.com.
"Anthony has long been one of the best defensive centres in Super Rugby, which will be a huge boost for our backline.
"Alongside Saia, the twins bring a formidable amount of experience to next year's squad."