Your older sister took the blame once so you wouldn’t. You probably never paid that back. You still can.

Somebody absorbed a punishment that belonged to you, and the chances are it was never discussed afterwards. Debts like that do not come with a reminder, which is exactly why they sit there for years. The person who took it has almost certainly stopped thinking about it. You have not, and the fact that it still comes to mind is the useful part.

There is no expiry on this. You can tell her you remember what she did, which is most of the repayment, or you can take something off her plate without being asked. A specific memory said out loud lands better than a general thank you.