Sebastien finished with five for 30 while fellow off-spinner Shane Shillingford picked up three for 45.
Both players shared the Man-of-the-Match award.
Set 20 to win, the Windwards lost Tyrone Theophile without a run on the board but left-handed opener Devon Smith smacked four boundaries in his 19 off 18 balls to see the visitors home.
The victory gave the Windwards their fifth win of the competition and put them on 60 points from their full complement of games while the Leewards finished winless with a mere seven points.
LEEWARDS 1st Innings 129
Windwards 1st Innings 268
Leewards 2nd Innings
(overnight 158 for nine)
K Powell c wkp James b Shillingford 51
S Peters c wkp James b Pascal 1
D Thomas c wkp James b Mathurin 28
*S Joseph c and b Sebastien 18
A Saunders b Shillingford 18
+J Hamilton c Charles b Shillingford 2
S Burton st James b Sebastien 19
H Walsh lbw b Sebastien 4
Y Leonard c wkp James b Sebastien 0
N Boland c Shillingford b Sebastien 0
Q Boatswain not out 2
Extras (b12, lb2, w1) 15
Total (all out, 59.1 overs) 158
Fall of wickets: 1-3, 2-65, 3-105, 4-105, 5-117, 6-144, 7-150, 8-155, 9-155, 10-158.
Bowling: Pascal 5-1-14-1, Gabriel 2-0-11-0, Mathurin 13-3-44-1, Shillingford 25-7-45-3, Sebastien 14.1-4-30-5.
WINDWARDS 2nd Innings
D Smith not out 19
T Theophile c Thomas b Leonard 0
J Charles not out 4
TOTAL (1 wkt, 5.2 overs) 23
Fall of wickets: 1-0.
Bowling: Peters 2-1-6-0, Leonard 2.2-1-6-1, Walsh 1-0-11-0.
Points: Windwards 12, Leewards 0.