Immediately before any payment, and again before the gift stage, because the program is recent and its published terms and procedures may change. Nothing in a secondary description should be relied on at the moment money is transferred.
Verify at the point of payment, not at the point of reading
Two moments in this process are irreversible and they are separated by months. The processing fee is nonrefundable from the instant it is paid, and the gift, once made, is not recoverable at all. Everything between them — vetting, adjudication of the underlying classification, visa availability — takes time during which a recent program can be clarified, amended or supplemented by procedures that did not exist when the family first read about it.
The discipline that follows is simple. Before the fee is paid, confirm the current terms on the official source and have a licensed United States immigration attorney confirm the household's position. Before the gift is made, do the same again, because the intervening period is exactly when a change would land.
Alongside that, watch the dates the family controls: the daughter's twenty-first birthday, the son's separate plan, and any deadline in Bertrand's business that assumes a departure date nobody has yet been given.