if you had 3 million dollars someone on the internet could go after your brother to retrieve it, if this didnt work use him as hostage to retrieve from you.
this kind of stuff can start with a simple drunk-talk.
some of the things that can happen:
your brother get drunk and talk to much, someone on the other table grab him and/or grab you.
your brother dont know the importance of cyber security and get stolen, you blame him or him blame you because none of you would know who fked up.
you do something that makes you insta-rich, he start thinking that you was getting more than you should of the money.
market crash and one of you wants to sell, other dont, if sell+up or dont.sell+down = family anger.
his girlfriend need the money for a surgey of a friend of her that steped on the paw of your doggo the day before.
i dont know how your family work, its just my perspective, i do invest and my sister started investing this year, i try to dont even insinuate in what or how to do it to dont take to me any responsability, because if things goes wrong, lets say, brains are hardwired to find someone to blame, i gonna step in only if a see a iminent risk.
personaly im a fan of paperwallets over ledgers, they are cheap, efective, can print hundreds and with copies to hide in multiple places even lots of dummy ones.
in the case of btc, armory have a feature of print a number of paper wallets that need another number of wallets to work together to have access to the funds, in that way you can print 10 and send 1 for each parent/friend, and need like 4 to work together so move the funds, but this can lead to the drunk-disaster i wrote and is too tecky.