I have finished in the Serpent Temple and I am now trying to figure out where the speaking oracle is to brief me on my next mission. The two others tell me to speak to their sister, but she seems to have disappeared.

The third oracle is in a tunnel leading from the circle of fires to a small room. You aren't supposed to get there but it is possible to do it :D
(Cast Lavawalk and push your back against the circle of fires. In the end, you will "jump" over the fires and occur inside the circle. Now, you can explore the area. Except the oracles, it contains nothing. Literally nothing. Moreover, the oracles are immune to any attack :D )
The oracles will not speak with you again. They will not acknowledge your deed. Therefore, forget them. You have their prophecy in your "logbook" so that you can read it again if you need it.