Devoneaux said:
I already spoke with him and have his permission to visit her.
1. When you say, "It states she's present in the castle," is this in quest notes? And, if it is quest notes, was it given before or after her brother changed faction?
2. When you say you have spoke to her brother, was this after he changed factions?
Looking at code she should respawn in one or other castles of brother's new faction, after her brother respawns in that faction. The brother needs to respawn to assign him a new home castle/town, which should be the new home of your lady.