Apart from that, my asset is my business and I have no obligation to declare them to any family member, for any reason, something is wrong with the guy in question and he should know that it is not right that give his family members such impression of being irresponsible to the point that if asking him to show them his proclaimed investment.
Probably the family members were just doubting him and that's why they wanted him to prove them wrong but since it's Bitcoin we're taking about here, there's no point of proving to the family members that he hodls a certain quantity because he isn't planning on selling them soon. Telling them would just put more pressure on him to liquidate his assets to assist the family when they're in need because it's when people know that you're rich that they begin to have sudden problems that requires your financial assistance. Keeping them in the dark was the decision but he should also put his life together so his family members don't have doubts about his financial capabilities. Because it might had just being, his family were concerned about him and only looking after his wellbeing.