My mom wants to tell my nephew the details about his university fund out of jealousy. How do I stop her?

English is not my first language, so I apologize for any mistakes.

I (26M) have been demanding my mother not to tell my nephew (12M) the complete origin of his university fund.

Context: My half-brother (HB) is 16 years older than me. After an unfavorable divorce for our mother, HB was raised by our grandparents, so they were extremely important for him.

When my father abandoned us, HB and our grandparents stepped up to help raise my sister (24F) and me. My grandparents even let us live in another house they own without paying them anything. We have lived there all my live. HB became a father figure to my sister and me. When our grandfather passed away, HB was the one who helped our grandmother stop her from losing the farm, paying out of his own pocket and doing a lot of the work alone without receiving any money.

Before passing away, our grandmother insisted on returning some of that money to HB. He refused at first, but she convinced him, saying the money was for his family. Only then he accepted and put that money into a university fund for his son (my nephew). However, because HB adored our grandparents and wanted his son to hold them in high regard, he told my nephew that the money was a gift left directly by his great-grandparents (that's is really closer to the true).

While going through our grandmother’s paperwork after her death, my mother discovered where the money came from. Now, she wants to tell my nephew that the money was actually a debt repayment to his father.

I strongly oppose this and told my mother to keep quiet. HB only accepted the money for his family's future and wanted his son to have a sweet memory of his great-grandparents.

But my mom insists on telling my nephew that the money is part of a payback for what my grandparents owed HB. My mother was always jealous of the relationship my grandparents had with HB, because for him they were like another set of parents, and he even called/calls them a variation that our language allows that is a little more affectionate than Mom and Dad, but my mom has always thought that they took her son away from her.

I can't understand why my mother wants to do this. HB's relationship with our grandparents benefited many people. My mother, my sister, and I live for free in our grandparents' house—they could have easily rented it out and gotten an income from it. All the work HB did for and with our grandparents kept them from losing the farm and today the city has grown so much that it's close to the farm, and its commercial value would allow all of my grandparents' children to have peaceful life for the rest of their days, if they manage it well.

I know HB will never forgive our mother if she ends up telling my nephew that.

What can I do? I already talked with my mother, but she refuses to change her mind.