okay in a few weeks time I am sending my iyanden force vs a 1500pt sisters of battle force

I am looking for tips / hints etc
I know they have anti psyker stuff so is my farseer going to be a waste?
could mind war kill something kill and limit his acts of faith?
whats the best targets
would prince yriel be worth taking?

From what I know, A Farseer is not going to be much help apart from aiding your Wraith Guard units. Perhaps take him for that alone, but do not kit him up to be overly expensive, or he may be a threat unto himself.

Yriel and the Avatar are both good ideas. They're not Daemonhunters, so as far as I am aware the Avatar won't be under a huge threat. Yriel is also nice because he has a ton of special abilities that will not be blocked by anti psyker stuff.

Watch out for assassins, especially the Callidus and Vindicare. These two are deadly and good at what they do! The Callidus poses a huge threat to Yriel and the Farseer, and you won't get a chance to shoot at it either as it will pop out of no where and assault! The Vindicare on the other hand will snipe off your unit leaders such as Warlocks and Exarch's.