Garbage. Most sisters won't have the same surnames after marriage for example. That's far too big a generalisation to make.
Mel_Traveller has made these sort of claims before. I have an enhanced family group that includes my wife (whose names isnt the same as mine), mother-in-law (same applies plus she lives at a different address in NZ) and my sisters (who's last names differ from mine and each other and also live in NZ). I have certainly noticed that when transferring to my sisters (who have married names and dont live at my address, in fact not in the same country) the transfer can be slow particular if I transfer from one sister to another using my card. One of the things I have done is add myself on their accounts as being able to manage their accounts and also added our family group members on everyones acounts as "travellers" so they have some sort of indication we might want to travel
with these people. In one of my sisters case we have actually met up so US could see that from our history.
So I dont think Mel_Traveller is wrong from the point of view that certain things may attact attention of the fraud dept of points.com or US Air, these include newly created accounts, accounts where the money is transferred to and immediately ticketed (in a 3rd name), different surnames are no doubt a factor as may be different addresses, in my opinion less important but I have no view of how US assess these things.
But I do have a strong problem with the idea that you are guilty until proven otherwise, the share promo encourages us to "Share miles with family and friends April 7 - 13, 2014" and also "Help friends or family reach their travel goals. Share your Dividend Miles with someone you love!". Are any of the conditions about not sharing to someone with a different names or addresses noted in the T&C's? No. If US does not want me to share miles with people who do not share my name or my home address they should not be inducing me to do so by encouraging me to share with family and friends. I have more relatives who do not share my name or address than those that do.
I am not personally that concerned about an audit, I'm a bit more concerned that on occassions US has apparently acted without contacting the affected FF members and then said we wont enter into any further discussion. So while I dont agree with the statement made by Mel_Traveller I do think it pays not to push the boundaries too much here!