If you were in his shoes, can you declare your crypto asset to your family? Or do you think he should have declared it to them after the extended family members must have left and even if he did, the information can still leak to the extended family and from there it can escalate to other people. What opinion do you have to give in this situation?
Myself like to keep things to myself and do not like it when someone questions me. As such, I would take sides with the guy and find it okay that he doesn't feel right to share his holdings.
If you are independent, you owe no explanation to nobody. Heck, even if you were dependent, pay people their due and you still owe no explanation.
You know your relatives better, so you should be the only one to decide if a relative deserves trust, the chances of he/she telling it to anyone else, and if it's worth the risk.
There is wise saying: if you want to keep something secret, do not tell anybody about it, no matter how much you trust the entity, the moment secret has left the tongue, that very moment it stops being a secret.