Okay but you know that if Sirius and Regulus had been alive/free and Harry’s guardians, they would have known that something was going on with Ginny almost immediately.
Like the Weasleys grew up in a household filled with Muggle items and rustic, charming, quirky, things. It makes sense that they wouldn’t have been able to detect the dark magic.
But Sirius and Regulus? Those two grew up in a household filled with all sorts of dark, twisted, objects. It’s pretty much a given that Walburga’s idea of interior decorating was using objects from Borgin and Burkes. They’d be able to sense dark magic from a mile away.
Like Ron and Ginny would visit Harry one day over the summer and Ginny would be like, “Look at my new diary; isn’t it nice?”
And Sirius and Regulus would just be like, “Yeah…we’re going to have to have a look at that.”
Not to mention the fact that Regulus would know what a horcrux is, so he would probably be able to recognize it. And there’s a 99% chance that they would recognize the name T. M. Riddle.
I can just imagine Sirius being like, “Okay, we’re going to invite Uncle Mad-Eye to this play date.”
Several minutes later, the house is filled with all sorts of Order members who are able to take the necessary measures to destroy the diary before it even becomes an issue.
Sirius and Regulus both buy Ginny a Muggle diary to make up for it.