What a mess, and unfortunately, this kind of scenario is far to common. The biggest risk is that whoever is in the house now refuses to leave, which is another nightmare altogether.
As far as I know, if the issue cannot be settled amicably, your option is to go to the judiciary. There, the first they they do is to insist upon a process of reconciliation. If that does not work, the judge will have the property valuated and it will go to auction where the owners have the first option to bid.
This can take years, and you will need to find someone here to act as your legal representative to push the cart along.
Thinking about the costs involved, it may make more sense to offer higher than the 15% value and hope that this will bring closure.
Good luck, and lawyer up.
"Hay que distanciarnos hoy para luego abrazarnos más fuerte".
-Guiseppe Conte, primer ministro italiano.