How is wealth, for which zakat has not been paid for over twenty years, to be purified, given the inability to determine the specific amounts of money on which zakat was due annually, and do you have an opinion on dividing the remaining balance among the siblings?

The father and uncles must pay the Zakat on their wealth for the past years if it reached the nisab (minimum threshold) and a hawl (one lunar year) passed over it. If they are unable to determine the exact amount, they should strive and investigate to ascertain it.

The father must hand over his deceased brother's wealth to his heirs and return the wealth of his other brothers to them if they request it.

As for the money paid by the insurance to the deceased uncle, any excess amount must be returned to the insurance company if it was commercial insurance, because it is illicit money (mal haram) and it is not permissible to benefit from it. If the insurance company does not accept it, it should be spent on public welfare. However, if the insurance was cooperative, there is no harm in taking all the money.