There is no way I'd use a web site for this, partially because there is no guarantee the site will still be here in 2 years, much less 20+ years. But more importantly, I can store that info in any document online. I don't need a new web site for it. I can (and do) also store it with the attorney who holds my will. And my older family members have a printout in a file cabinet at their home where I know where to look.
Overall, this is not a tech problem. You are trying to use a web site to replace a communication gap with your family.
Overall, this is not a tech problem. You are trying to use a web site to replace a communication gap with your family.