Eva - Thanks for the question and interest in my blog!
Your question regards the role of a first sergeant for rooks next year. I was a staff sergeant for rooks this year which means i work along side 2 other staff sergeants, and under a platoon sergeant and platoon leader to make up out platoon. There are 3 different platoons in a company. There is one first sergeant per company who look over all 3 platoons. Their job is to make sure the information gets passed down correctly to each platoon and to make sure each platoon is being trained correctly. They are also in charge of administrative paperwork, such as DAFs (when rooks get in trouble), Class 1's (when rooks get in a lot of trouble), Counseling statements (positive or negative), and other forms.
I have been lucky this year to work with a fantastic 1sg who is going on the be the regimental XO (2nd in command of the corps). I'm sure your relative will do a fine job because these are very competative positions and (usually) only the best of the best are selected for the job.
Again, thank you for your question!