Assalamu alaikum - Inheritance being denied, need advice

Assalamu alaikum, brothers and sisters. May Allah have mercy on my parents and grant them Jannah. My late father had a share in an ancestral property that was always thought to belong to our grandfather. The understanding in the family was that the property would be shared informally among the 2 brothers and 2 sisters (including my father), with no paperwork or formal partition ever done. During his life my father tried to sell or develop his share but never finished the process. There was never any argument between the siblings about their portions. When he passed away, I took responsibility for handling things for our branch of the family. While my grandmother was still alive I tried to proceed with disposing of the share. Again, there was no dispute and everyone was aware of what belonged to whom. My grandmother knew of this arrangement and raised no objection. But recently we discovered that the ancestral property had actually been split on paper into two parts - one in my grandfather’s name and one in my grandmother’s name. Now that my grandmother has passed away shortly after, my family is being denied inheritance from the portion listed in her name on the basis that my father predeceased her. If this had been a hidden fact all along I would accept not getting that share. But this was always understood informally to be my family’s portion, and my grandmother seemed to have no practical control over it. It feels wrong that we are being refused simply because there’s a document with her name on it - a document she probably didn’t even know about. How is this fair? It seems like pure chance we found out that my grandfather put the title in her name. The siblings never objected before and everyone knew their shares. Can anyone please advise what I should do or how to approach this? JazakAllah khair.