Is it permissible for the eldest brother to take his siblings' share of the inheritance to invest it in an unguaranteed project abroad without guarantees for the return of their money, and is this considered an injustice and a severing of family ties if the siblings refuse?

If the elder brother takes another's share of the inheritance without their consent, that is an injustice. And his withholding of documents proving his younger brother's right to the inheritance, after they have been requested, is an injustice and a denial of rights.

Inheritance documents must be made available to every rightful heir, or their contents must be authenticated in a way that guarantees these rights.

As for the younger brother's refusal to give his share to his brother, that is not an injustice; for everyone is more entitled to their own right, and a person's demanding of their right is not an offense.

If demanding it results in the severing of kinship ties, then the sin of that falls upon the one who severs them.

And if the children of the younger brother are in need of this money, they are more deserving of their father's kindness than their uncle, especially if the uncle is well-off.