After completing the quests in the first town (crypt, toad, rogue camp, windmill, etc.) I head to the cave. The oracles will tell me about turning west, hint at the next quest, then tell me that first I must go to the serpent lair. If I try to talk to them, they tell me to "Speak to my sister". I do not know what they mean and they won't open the passage. Any help would be appreciated.

Make sure the person doing the talking has the signet of the Mavin, and also, that you are clicking through all of the potential conversational threads.

The Wizards and Warriors Story Archive

talk to the one in the middle the other two tell you to talk to her (their sister) and folow the advice above