<p>Look at your thread title: “interfere” is the operative word. If I were the cousins, to receive a letter out of the blue from a disconnected family member, telling me I’m not really doing a great job as a child, would not improve matters 
No matter how neutrally you word it, they are bound to be defensive. And in all honesty, you admit to having no idea how this family became estranged, or why. This is not your business nor your problem to solve. It’s theirs. If you meddle, it could make it worse, not better. </p>
<p>That being said, it is lovely for you to support this man as he ages. Let that be enough. If he is meant to reconcile with his children, that will happen in time, but I would leave well enough alone.</p>