The reasoning is straightforward: a lender wants to know the down payment is truly a gift with no expectation of repayment and no undisclosed second loan sitting behind the purchase, since either would change the borrower's real debt load.
A gift from outside the immediate family, or one with any repayment expectation attached, is not treated as a true gift and can complicate or sink the mortgage approval entirely, so the relationship and the paperwork both need to be right before the funds move, not after.
Joel Dyck can point a buyer to a mortgage broker who will confirm exactly which gift sources their specific lender will accept before an offer goes in. Get your hand-reviewed valuation from Joel Dyck.