Usually before, if the sale would change the proportional alignment the affiliate claim depends on. The relationship must exist at filing and continue while the transferee holds status, so filing on a structure that is about to change invites trouble at extension.
File on the structure you will still have
Hypothetical example: the Hampton siblings intend to sell fifteen per cent of the Canadian company to a long-serving shop supervisor next year, and are considering filing the petition now on the current fifty-fifty structure. The petition might well be approved, and the problem would arrive at the extension, when the two ownership pictures no longer align and the affiliate claim has to be argued on changed facts with a family already settled.
Where a change is planned and reasonably certain, completing it first and filing on the resulting structure is the steadier course, even at the cost of a few months. Where it is genuinely uncertain, file now but record the possibility and take advice on what the change would mean, so the decision is made deliberately rather than discovered.