Well you weren't supposed to kill the Abyss Watchers, but as long as you didn't kill the sage then you should be fine.
If you already happened to kill both the area bosses, then you're out of luck. But next time around, don't kill either of the area bossed (Crystal Sage and Abyss Watchers). Instead, after leveling up 5 times from Yoel, kill him. Then rest at the bonfire and return to his body; Yuria should be there now. If somehow she isn't there, you may need to teleport somewhere and back (or just use a homeward bone/coiled sword fragment). Now kill Yuria, and the Pale Shade should be start invading you.