An eight-wicket win over the Thunder at ANZ Stadium on Saturday night lifted the Sixers above the Perth Scorchers to second on the ladder.
Perth can regain the higher position and home-ground advantage for their February 5 semi-final with a victory over the unbeaten Melbourne Stars at the MCG on Monday.
The Thunder collected their third wooden spoon in as many BBL seasons and have an horrendous 3-20 win-loss record over that period.
The Sixers made it five wins from as many matches over their local adversaries, reaching their target of 129 with four balls to spare.
Nic Maddinson (59 not out off 56), who was dropped on 13, and stand-in skipper Moises Henriques (27 not out off 30) sealed the win with an unbroken stand of 77.
Boosted by a T20 equal career-best performance from Test spinner Nathan Lyon (3-14 off 4), the Sixers restricted the Thunder to 6-128 after inserting the home side.
However, Lyon will be unavailable for the finals due to his Test commitments, while fellow tweaker Steve O'Keefe is in doubt for the Perth game after dislocating his shoulder.
He will have scans on Sunday.
"It was a weird feeling landing on the ground. I couldn't move it, I wasn't quite sure what I'd done," O'Keefe told the Ten Network.
"Fingers crossed it won't be too much time off the field."
Sixers lost English import Michael Lumb (2 off 6) in the second over but always seemed to have the chase under control despite leaving it to the last over.
Mark Cosgrove (32 off 24) partially atoned for his part in Lumb's run out with a breezy cameo.
In a depressingly familiar situation for long-suffering Thunder supporters, their brittle batting order again failed to deliver.
With Usman Khawaja ruled out with a hamstring injury that sidelined him earlier in the competition, Thunder didn't get big the runs they needed from star batters Tillakaratne Dilshan (2 off 9) and Michael Hussey (21 off 18).
Dilshan completed an underwhelming series with 62 runs at 15.50.
Hussey produced a thrilling pull for six off Brett Lee (1-35 off 4), but the innings went into an irreversible tailspin when he was the second man out with the score at 60 in the 10th over.
He was caught at deep midwicket by Abbott off Lyon, who four ball later bowled a charging Daniel Hughes (32 off 33).
Kurtis Patterson (27 off 26) and Ryan Carters (15 not out off 10) at least ensured the Sixers would need to score at more than a run a ball.