Sinclair was selected to British Columbia's under-14 girls all-star soccer team at age 11 and led club team Burnaby Girls Soccer Club to six league titles, five provincial titles, and two top-five national finishes. She attended Burnaby South Secondary School where she led the soccer team to three league championships. At age 15, she attended matches of the 1999 FIFA Women's World Cup in Portland, Oregon. She played for Canada's under-18 national team before making her debut at the senior level at age 16 at the 2000 Algarve Cup where she scored three goals.