Siakam posted his 10th straight and NCAA-leading 18th double-double. Ian Baker added 15 points and Jalyn Pennie 12 for the Aggies (12-8, 4-1 Western Athletic), who made 22 of 26 free throws to Seattle's 6-of-13.
Siakam's basket with 12 minutes left put Aggies up by 15, 43-28.
A steal and basket by Cohee got the Redhawks within five, 65-60, with 30 seconds left but Baker made two free throws and Braxton Huggins made a steal and hit 1 of 2 free throws, getting the rebound after his miss.
Two 3-pointers by Pennie gave the Aggies a 26-22 halftime lead.