FUN FACTS:
1. To revive Sassan we need to bring back her a ring. Yrp, that's all, simple ring and - voila - dead walks again.
2. To revive Ygerna we need to finish the game, kill tons of enemies, become a dragon, etc, etc...
Fun isn't? :P
And yeh, don't try to find any sense in game's plot - there is none.