Not the problem you are thinking of.

The real problem is, if you were hit by a truck tomorrow, who would give the money to your niece/nephew?  

It's better to sign a promisory note (debt note) saying you borrowed this money from your brother and if you die the money goes back to him or his kids. 

The same promisory note can be used to show IRS that you didn't earn this money, in case they are checking on you. 

$700K is a lot of money, you guys are so careless. 
 

Another problem is that estate tax for foreigners starts at $60K.  Anything over $60K is subject to 40% estate tax.  Think it over.