what would have happened if luke went to the dark side?

He did turn to the darkside in one of the Dark Empire novels, Leia brought him back though but i think he struggled with it for awhile after.

That would've been a sad,sad day.

then order 66 will finnaly be complete

The Jedi would have been completely destroyed and the Sith would have won. Even if Leia received her proper training, i doubt she could ever be strong enough to defeat both Luke and Vader