Sciver previously played for the Stars in the first two seasons of the WBBL before switching to the Perth Scorchers while Brunt also has past experience with the Scorchers.
Sciver has scored 952 runs at 24.41 with a strike-rate of 112.52 across 52 WBBL matches alongside claiming 36 wickets. Brunt, who didn't play in last year's tournament, has taken 49 wickets at 17.71 and an economy rate of just 5.15 in 44 games.
At the T20 World Cup earlier this year Sciver was England's leading run-scorer with 202 in four innings including three half-centuries.
"In Nat and Katherine we've got two of England's most successful cricketers who have been part of the WBBL since the tournament began," Stars head coach Trent Woodhill said. "We're pleased to have Nat coming back into green again after a couple of seasons in Perth and Katherine's experience and bowling pedigree need no introduction. It really enhances what is already a good team coming together."
The England pair will be part of a Stars line-up that includes Australia captain Meg Lanning as she made the journey back from the Scorchers. There is one spot left in the 15-player squad with the Stars due to announce a final signing next week.
The WBBL, which will start on October 25, will be played entirely in Sydney this season due to Covid-19 travel restrictions.
Current squad Meg Lanning, Katherine Brunt, Nat Sciver, Elyse Villani, Mignon du Preez, Annabel Sutherland, Lucy Cripps, Alana King, Holly Ferling, Erin Osborne, Tess Flintoff, Georgia Gall, Sophie Day, Nicole Faltum.