Spoiler
If I remember it well, the oracle was never real. Tia fabricated it to get Lysia to marry her brother. The end goal was to have the support of the people, I think, since she is a saintess and is very loves by everyone, and her brother is a bastard son, from their mother who has a terrible reputation. So, the whole "she will become the empress" is not real.
I havent read the novel but based on context clues, I think Tia meant that she manipulated everything to make a fake oracle so Lysia can marry Lawrence to give him the emperor's seat. Lawrence needed Lysia, the Saintess, to legitimize his position as crown prince.
Oh ok thanksss
So what will happen to the other girl, Felice or whatever. Who will she marry to be empress? If she gets the oracle, does that mean Tia can't make Cedric the emperor because she decided to stay married? Is the oracle fake because the white haired dude made it up to marry her? Will the white haired brother still be emperor? Can the oracle change?