When in doubt....go to the horses arse.....er... mouth.
ATF FORM 4473 (Firearms transfer) Page three, paragraph one:
1. For purposes of this form, you are the actual buyer if you are purchasing the firearm for yourself or otherwise acquiring the firearm for yourself (for example, redeeming the firearm from pawn/retrieving it from consignment). You are also the actual buyer if you are acquiring the firearm as a legitimate gift for a third party. ACTUAL BUYER EXAMPLES: Mr. Smith asks Mr. Jones to purchase a firearm for Mr. Smith. Mr. Smith gives Mr. Jones the money for the firearm. Mr. Jones is NOT the actual buyer of the firearm and must answer “no “ to question 12a. The licensee may not transfer the firearm to Mr. Jones. However, if Mr. Brown goes to buy a firearm with his own money to give to Mr. Black as a present, Mr. Brown is the actual buyer of the firearm and should answer “yes” to question 12a. (emphasis mine)
I see no restriction to 'family member' to this.
In fact, several years ago my best friends sister wanted to buy her husband a Ruger super blackhawk(?) .44 mag revolver for hunting. Unfortunately she was only 20 at the time so she had to have her brother do the paperwork.
Even though he passed the revolver off to his under 21 sister the moment he got it, it was still regarded as a legal (read: not 'straw') purchase because it was a legigitmate gift to her husband.
