Here is the prompt:

"Write a cover letter for your sister-in-law who will be applying US Visa at US embassy in India to join you in your commencement, which will be held in May 10, 2019. A good cover letter will help your sister-in-law in getting US visa. So, write it as good as possible. Note: This letter is for US Consular officer who will give Visa to your Sister-in-law. So, write the letter, which will convince the US consular to give her US Visa."

I have no idea how to write a cover letter for US consular. Please help me.