All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Your uncles do not have any right to demand what your grand grandmother had donated to your brother, if the donation was Islamically proven while this grandmother was in good health and she could dispose of it as she wished.
This is because this gift is a disposition of an owner in his property when his disposition was effective and there was no legal impediment about making it effective.
Your grand grandmother's donation is not considered as being unjust to the children when gifting them because the grandson is considered as a stranger due to the existence of direct children; the evidence about this is that he does not inherit and it is permissible to make a will to him.
Therefore, it is permitted to donate to him like any other stranger and the sons do not have any right to object to it.
Finally, we would like to draw your uncles' attention that they are supposed to be kind and helpful to the members of your brother's family after his death and not try to take from them by force what they already possess.
Allah knows best.