Eventually, the party will figure out that Lucius has been disowned and is actually the villain in this story.
They could try to bring him to justice, but the justice available locally is nowhere near what we would call justice today. In fact, it would be pretty much their word against his, and they are not only foreigners to this place, but would also base their claims on "we saw ghosts". If the characters are honest with themselves, they would rate their chance of bringing Lucius to justice this way rather low. Equally likely they would be arrested and executed for being satanists who converse with the dead.
If the party insists on bringing Lucius to justice, let them set up their argument and resolve it with a roll on social. Their position for this roll is desperate and the effect is limited. They will have to bring their best game to win this one.
The best alternative is to find Gregorius and bring him in as the real heir, especially if they found the father's last will. A man's last will is highly respected and both the villagers and the local warlord would enforce it. However, this requires them to bring the real heir forward, as to the best of everyone's knowledge, he perished in the fire.
There are a number of family portraits that the party can recover in the villa, namely in the Inside - West > Apsidal Hall und Inside - South > Exedra. They can also ask around in the village as the family was well known.
Some scrying magic can find the brother, or they can search and ask around in nearby villages, which will take a few days but will eventually succeed (you can handle it as a long-term project if you want).
Why would Lucius sent the adventurers into the ruin if they can uncover him? Because he doesn't know what the ghosts show. He believes that the ghosts are just apparitions.