so I kept wondering if an investor can invest in bitcoin for a long term plan without disclosing to any of family members?
An investor can let a family member know that he is investing in bitcoin, but he shouldn't tell them the worth of bitcoin he has in his wallet. And the investor shouldn't tell every members of the family. He can either pick one member of his family who he is very close to and let one know about it, but he shouldn't make the mistake of revealing how much his assets worth to that person.
Is it proper sharing your pass phrase to your family members?
No it is not proper to share your phrase with anyone, be it family members or friends. Because if you do it's the same thing as shooting yourself in the leg. Secondly Don't put all your money in bitcoin, as you are investing, make sure you have a good fiat reserve as well so that if anything happens to you your family can fall back to that one and use that one to fix things. The mistake most investors do is putting all their money in bitcoin which is wrong. Write your bitcoin phrase down and store it in a place you know your family can only access when you are dead. Until you are dead nobody should access your wallet phrase.