a parent or legal guardian can buy a gun for a an adult under their care under 21 in texas. your sister is probably not your mother or legal guardian so no she cant. she cant gift it to you either until you turn 21. the law is the same for private party buys.
my opinion is get the money put it away until you hit 21 then buy it yourself. that is unless you hate your sister and want to send her to prison. course you would prolly go too unless your a registered democrat and obama supporter. just yell "eric holder did it too!!" while they haul ya off
a straw purchase has nothing to do with whose money it is or where the money came from. it is whether or not the person getting the gun could or could not legally buy one. since i can legally buy a gun anyone else that can legaly buy a gun can do so and give it to me. thats why private sales are generally legal to do as is gifting. if you buy a gun wait 15 years then gift it to a person who cannot legally buy a gun that is a straw sale.
it is up to the seller to determine if the buyer is legal. its also why i dont do private sales without using a ffl.
since the op cannot legally buy one his sister would be straw buying regardless of where the money came from.