when is part 1 of family secret revealed?

I'm trying to decide when Ash's sister tells him what she found: that he was obviously the product of an affair. I already wrote that conversation as a flashback. in the story, he should actually have known it since his college year, because his sister, Cami, gets in a fight with their mom and and brings that up. But Ash doesn't remember that until he finds himself facing the same temptation.

I thought to delete that and have Cami, in the present of the novel, converse with Ash, reminding him of that fight and the knowledge of his mother having an affair. But then the question becomes--is it more effective to the narrative to have that occur before or after Ash is in his situation? It definitely changes the impact. One way, it's foreshadowing--but it also should make Ash think twice, be less likely to succumb. His remembering after, when he's dealing with regret, makes more sense--that the memory comes to his mind only because he repeats the mistake.

So maybe I shouldn't get rid of my original concept--I should keep it as a flashback that comes, bidden when he is regrets his choice.