We have to see both sides of they story, there can be two scenarios here, one is that he just ignored or we can say forgot to share hos passwords and key with his family or the second scenario could be that he intentionally never shared the key, may be he does not want to give his asset to anyone, who knows.
According some reports he was addicted to drugs and maybe that contributed to him neglecting his duties about making a proper will and ensuring his family are financially looked after.
Give them your private keys.
Do you see any other solution? There isn't any.
Either you will give them your private keys or not. Maybe to store them in safe but still they will have to know safe password.
You can give private keys to family but that means they can access your funds during your lifetime.
It's possible that he bought them a long time ago when it was peanuts and never thought it would be worth anything. Hence he didn't bother to tell his family because they wouldn't understand how it worked and it wasn't worth much at the time (relative to his wealth).
Maybe he did buy them when they were very cheap but he must have known their value after all he was according to reports spending huge amounts of money of drugs (though there is no real evidence about that)
If he had $1 billion worth of XRP then at $0.50 each he must have had around 2 billion XRP.
The current total XRP circulation as of today 4th June 2018 is 39 billion so that means he alone was holding over 4% of all XRP. Those are staggering numbers if true.