Is inmate aid legit? Will my sister get the letter I wrote?

Absolutely, 100% as long as you selected the correct facility. But even if there was a mistake, or the inmate got transferred, InmateAid will resend the mail again at no charge. The Company has served over fifty million users since 2012. 

Read the testimonials, there was one that should give you great comfort: "My nephew was recently moved from a facility to a new one but I had just sent an electronic postcard to the old one & inmateAID forwarded it to his new facility without me asking. I was very pleasantly surprised & grateful! Thanx so much!"