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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?
I would also do the same thing as that person, because it is a private matter, besides the ones who are wrong here are the other family members, why should they know the bitcoins that the person currently owns, do they want to show off or want to belittle the man!
As a sibling, we should not interfere too much in the affairs of other family members, because there are many types of people and many of them never want to share everything with other family members, help as needed but don't interfere in everything.