I'm sure Corinne does know, or at least has narrowed down to "either of my stepsisters".

But LW's parents don't*, and at any rate, LW should still apologize from the heart. (But not use that apology as some sort of wedge to open up contact! Keep it short and sweet - I did this awful thing, I'm so sorry, I hope you can forgive me but I understand if you can't - and done.)

* Or are claiming they don't? Or anyway are acting like they don't? Do they seriously, after all this time, still think it was Corinne who is just stubbornly refusing to confess?