Yes, Tia means aunt.
If you go back and look more closely at the lady's name, you'll see "tia" right smack in the middle of it. That's probably how she got the nickname, from her own legal name. Look at each letter very closely.
I just saw someone answered you already, I think it was tx_dot.
I find this thread more than fascinating.
Casey and her family must have played the clues game before. They all seemed to know what she was doing, even though they didn't catch on soon enough to run out there and move that body. I can't imagine real grandparents moving and hiding the body of their little grandchild just to save a murderer. Since the As are a family for the books, I'm sure they are kicking themselves now for not catching on sooner. Did it take them until November to figure it out? hmmmm? Did they or one of them send the PI out there? Sure seems that way. Great possibility.