Hi Russell, and welcome
Perpetual licences can be confusing, especially with how Adobe changed things over the years. A perpetual licence isn’t actually a lifetime subscription. It means you buy the right to use that version forever, but only on a computer and operating system that still supports it. Once the hardware or the operating system moves on, the old software often stops working. Adobe doesn’t reactivate those older versions anymore, so you can’t reinstall CS6 through official channels today.
There is still a way for you to get at your files though, so don’t worry. You or a family member can install the current InDesign on a free trial. That will let you open your old CS6 files, look through them, and export anything you want to PDF so they can be shared or archived. The trial runs the full program, nothing is locked out, and you don’t have to subscribe if all you need is to access your older documents.
If you still have the original CS6 installer discs or serial numbers from years ago, those technically remain valid, but getting CS6 to run on modern systems is very hit and miss. The free trial route is usually the simplest and least frustrating.
Hope that helps you and your family revisit those memories. If you get stuck at any step, feel free to ask.