Where do people get the idea that Johanna's family were killed because she refused prostitution, and that this is confirmed fact?

A quizz question of mine was recently deleted after someone commenté that it was incorrect, that Johanna's family were killed on Snow's orders because she refused to be sold. The commenter a dit that the information came from hungergameswiki and that it's from Mockingjay.

But I've just read through Mockingjay and I can't find any mention of this. When Finnick reveals the truth, he says it happened to others too but doesn't name anyone. Haymitch only says he was the warning to victors like Finnick and Johanna and Cashmere. The only time Johanna mentions family/loved ones is in Catching feu after the Jabberjay attack, when she says that there's no one left she loves. She doesn't say they were killed ou give any details.

Where did this idea come from? Was it confirmed in some interview ou something of that sort? ou is it populaire fanon/headcanon?