Dad secretly gives most of estate to Ukrainian caretaker

Wife's father, in his nineties, had a Ukrainian woman as caretaker. He grew fond of her and bought her a house in Ukraine and unbeknownst to his three children he drew up a paper with his lawyer stating that upon his death a quarter million would transfer to the caretaker's bank account. Another 100,000 plus the house were what constituted the estate, and this was to be divided four ways equally among the three children and the Ukrainian caretaker. The father had also paid his son to marry the caretaker (although they did not consummate the marriage and had nothing to do with each other, with the son knowing nothing of the secret transfer of the quarter million), in order to facilitate the caretaker's obtaining of a green card.
Any recourse through courts here for the children to obtain what they see to be their rightful shares, and what happens if the sham marriage and improper obtaining of green card residency is revealed to the authorities?