In a crucial match for both teams, the Heat's bowlers delivered to restrict the Sixers to 8-127 at North Sydney Oval, defending their total of 9-139 with apparent ease.
Led by Jess Jonassen (3-24), the Heat remain fifth on the ladder with a 4-3 record after two straight wins.
The Sixers, however, are at risk of missing the finals for the second straight season with a 3-4 record while sitting sixth on the ladder.
Even more concerning for the Sixers is the fact they were somewhat in control midway through the match.
While Grace Harris (39 off 26) and Georgia Redmayne (28 from 23) threatened to get away for the Heat, Amelia Kerr's 4-23 had the Sixers well on top.
The New Zealander spun her way through Brisbane's middle and lower order, while Sophie Ecclestone also claimed 2-18 from her four over.
But from there, it fell apart for the Sixers on a ground where runs are generally easy to strike.
Alyssa Healy's difficult start to the tournament continued when she skied Shikha Pandey (2-25 in the first over, while Ellyse Perry also fell for one early.
Kerr became Pandey's second victim shortly after, and when Ashleigh Gardner was lbw trying to scoop Jess Jonassen, the game was all but over.
Sarah Bryce (30 off 26) tried to stage a comeback but her dismissal ended the Sixers' hopes. (Bianca De Marchi/AAP PHOTOS)
Sarah Bryce (30 from 26) and Sophie Ecclestone (24no from 18) threatened a late comeback, as teenager Caoimhe Bray also helped get the equation down to 29 off 12 balls.
But Pandey and Jonassen stood tall when it mattered in the final two overs to get the Heat home.
The other point of note for both the Sixers and Australia is the fitness of Healy.
The 34-year-old opted not to keep on Thursday, with Sarah Bryce instead taking the gloves.
Healy has previously said she expects to be managed through the summer after a foot injury in the World Cup, with Thursday's move part of that.
"I've got a few things going on at the moment that I am managing," Healy told Fox between taking two catches at short third.
"It's obviously a really big summer of cricket. I am still really confident I can bat, I just wasn't sure about squatting, which is one of the concerns.
"I am just trying to manage that, get through WBBL, we have six ODIs before Christmas and then the Ashes."
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