The Heat, chasing the Sixers' 140 all out, made 5-142 from 17 overs for victory at Allan Border Field on Sunday night.
But the Heat needed to chase down their target in 13.3 overs to eclipse the Renegades on net run rate and secure hosting rights for the final.
The Renegades (14 points) will host the WBBL decider at the MCG next Sunday night after topping the table from the second-placed Heat (14 points).
The Brisbane outfit will host the Challenger on Friday night at Allan Border Field against the winner of Wednesday night's Knockout final between the third-placed Sydney Thunder (13 points) and fourth-placed Hobart Hurricanes (10 points) at Drummoyne Oval in Sydney.
The Heat on Sunday night made a dash for top spot in the last game of the tournament's regular season.
No.3 Jemima Rodrigues smacked five fours and a six in making 39 from 22 balls but her rapid-fire knock ended when caught from Melie Kerr's bowling and the Sixers spinner bowled Charli Knott next ball.
The quick wickets left the Heat 3-58 after seven overs and, met with some miserly Sixers bowling, they changed tack to focus winning the game, rather than chasing the net run rate target.
Georgia Redmayne made 25 from 29 balls and the outcome remained tight when the Heat required 46 runs from the last 42 balls.
Brisbane captain Jess Jonassen then called the power surge and capitalised on the field restrictions by cracking three fours and a six as Caoimhe Bray conceded 20 runs from her over.
The burst from Jonassen (39no from 29 balls) and a late flurry from Laura Harris (12no from five) sealed victory.
Earlier, the Sixers couldn't cash in on a bright start from their opening batters Amelia Kerr and Ellyse Perry.
The duo put on 63 runs before Perry (33 from 34 balls) was dismissed in the 10th over.
Ash Gardner (three) became Lucy Hamilton's first victim some two overs later and when Kerr's 37 ball innings ended in the 14th over, the Sixers had slipped from 0-63 to 3-90.
From then, only Sarah Bryce, who smashed a six and four fours in making 28 from 19 balls, reached double figures.
The Heat's emerging 18-year-old star Hamilton (3-21) was again superb - she now has taken 10 wickets in her past three games - while skipper Jonassen claimed 3-26.
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