Sixers seamer Sarah Aley earned Player of the Match honours with a four-wicket haul that took the match away from the Strikers.
Chasing 139 to win, the Strikers suffered a horror top-order collapse, losing 6-3, and made 9-121 at Adelaide Oval.
The Sixers will seek consecutive titles when they play Perth Scorchers in Sunday's grand final - a rematch of last year's decider.
The Strikers lost their initial six wickets in an 18-ball span to cruel their run chase.
Aley took four of those wickets, starting the rot by snaring wickets with her first two balls.
Teammate Kim Garth claimed two wickets in the next over before Aley made another double-strike in the following over.
Aley had stunning figures of 4-1 after her initial two overs - she finished with 4-18 from four overs.
Earlier, Sydney's Ashleigh Gardner blasted a half-century from just 31 balls as the Sixers posted 5-138.
Gardner finished with 72 from 45 balls in a knock featuring six sixes and five fours.
The powerful right-hander helped Sydney recover from the loss of opener Alyssa Healy (two) on the ninth ball.
After a cautious start - the Sixers were 1-9 after four overs - Gardner launched as captain Ellyse Perry (16 from 31 balls) played the anchor role.
The Strikers stemmed the flow and conceded just eight runs in the final two overs, with Sophie Devine taking 2-21.
But Adelaide's batters then spectacularly collapsed: from 0-20 after three overs to 6-23 after six.
Tammy Beaumont (50 from 46 balls) offered resistance but the Strikers fell well short of advancing to their first grand final.