When I first read the Birthright game book I was glad to see the Sidhelien as an angry lot who keep themselves apart from other races.
I disliked the idea of Grey, wood, sylvan, high, etc... wanting to live in human cities and swear fealty to humans.
I felt the Sidhelien were a very proud race steeped in history and tradition (they are after all, immortal).

Unfortunately I do not own the PSB for Tuarhievel, do they give much in the way of sidhelien society and customs?

I would hope that Sidhelien wizards have created some healing type spells as there are no priests in their society.
Sidhelien must also have excellent herbalists. Excellent craftsman for that matter!!