Sin’dorei is a political designation, not a genetic trait.
“Sin’dorei” means “children of blood” and was the name Kael’thas gave to his people in honor of the fallen and has nothing to do with their usage of fel.
Kael’thas, sometime after renaming his people, went to Outland and brought back fel crystals, to aid in sating his people’s addiction to magic. This stopped their withering, but had the added effect of being even more highly addicting, causing the problem of the wretched who over indulged on fel crystals.
A Sin’dorei could be highly loyal to Silvermoon, not take in Fel crystals, retain their blue eyes through a means of meditation, and still be in Silvermoon.
A Quel’dorei on the other hand, is a group of Elven kind who deserted Silvermoon and went scurrying over to the Alliance, even after their treatment of them in the wake of the Scourge.
Ren’dorei are Elven kind who took void baths and shop at Hot Topic.
Kaldorei are expired, and at this point, a bit crispy.