I think it's likely that it could be a Cloned family member problem. I had a very similar problem where the faction heir for Macedon was duplicated. To get around it, I FDed the settlement he was in off the makedons, but you can't really do that here.

I guess the only way is to move_character an FM there and besiege it with just the general. This will make them sally, and if you do auto_win defender each time, they will sally until you eventually take the settlement. If you are having trouble finding a general who would work for the command, just use the command on each family member, using only their first name, until you get to one that works. There's usually a few Family members who can be teleported if you only use their first name.