Do the heirs of a grandmother demand only the value of the original inheritance, or also a percentage of the profit generated by the trade in which this inheritance was invested since 1964, after the death of the grandmother's brother who seized the inheritance?

Your grandmother's right to inheritance is established and cannot be forfeited by prescription. You have the right to claim the inheritance and a share of the profit. Scholars have analogized this to the case of someone who trades with a deposit entrusted to him, or someone who seizes another's money and trades with it, and the correct view is that the profit is divided between the two parties.