Can my sister under age 21 bring me back alcohol to the United States from Germany when she returns? Would there be any issue? What if I met her at the gate?
No, this is not legally possible. From Bringing alcohol (including homemade wine) to the U.S. for personal use:
It is illegal for travelers under the age of 21 to import alcohol - even as a gift.
You cannot meet her "at the gate" because she must pass through customs (where she must declare the alcohol) before she has an opportunity to meet you.