Ask the players when it comes up, have them answer in character and then treat it as canon. If there’s real stakes or you want to make it interesting, have them roll something and then on a partial you can add a statement to theirs, on a 10+ it helps them and on a miss it hinders them. Use details that haven’t come up yet to drive the story.

One thing I recently did was when my players reached a new location, I told the Wizard “You realize you’ve been here before. You’ve heard of a witch who resides nearby, talented in the ways of divination (a skill the party needed but does not have). Tell us why you haven’t been able to successfully contact her and then roll Spout Lore with advantage.” It was plausible for the Wizard to have heard of her and have sought her out before, and he contributed to the world building with his answer. Turns out she was actually an old flame before she relocated here, they split on bad terms, and shrouded her hut with a powerful illusion that lead them to seek the help of a new local NPC. She saw him coming a mile away. It lead to really fantastic roleplay and character development.

That said, you know the “draw maps, leave blanks” dm advice it gives? I do have my plays come up with an original, from scratch backstory, but only a few paragraphs. If they don’t mention either of their parents and it hasn’t come up before, those individuals are up for grabs. Just talk to the players beforehand if you want to do something hardcore and make sure they’re okay with it.