why does fire resist, invisibility or fire resist potions not stop winski killing my lead character at the end of BG. and why does invisibilty purge not detect Winski or Angelo?????????

they could be wearing a cap of non-detection

The trick is this:

DON'T attack Sarevok before all of the dopplegangers are dead. Sarevok will then speak again and afterwards turn hostile. NOW you can attack him with RANGED weapons and summoned monsters, whereupon Winski will appear to teleport his master out of the room, leaving you well alone.

If you feel like attacking Sarevok just like that you'll get nuked. Period.
Remember: You're the good guy (or girl)