Hmmm.  This raises an interesting question.

When an investor buys 5 shares, the fund has to buy one bitcoin.  But where will the fund go to buy the bitcoin--from the open market, or from the twins?

If the fund buys bitcoins from the open market, then it should have a very positive influence on the dollar equivalent value of bitcoin, which we'll all enjoy.

But what if the fund buys bitcoins from the twins?  That will allow the twins to offload their bitcoins without affecting the market.  That will be good for them, but it won't drive up the dollar value of bitcoin.

I don't think there's any legal issue.  But it'll be interesting to see what the exchange rate does when COIN finally opens for trading.