If you have done your training miles you will be fine.  The best advice I got for my first half is to not start out too fast, use your first few miles as warm up.  The most important thing to remember is to have fun and make sure to drink plenty of water and carboload the day before. 

Wonderful advice from the others...I can only offer to find your personal comfort zone (others will get you off pace in one way or another and it is easy to find yourself going too fast or too slow along with a group), so try to tune into that special mental mode of where you want to be to move well and finish. 

P.S....this is your first race?  Well, now you will have to implement your own version of the reverse taper and do a 5k, 8k, and 10k (in that order, of course), as part of your appropriate recovery efforts from the half.  !

Seriously, races are great opportunities.  Congrats to you and your niece...you all should have a wonderful experience and wishing you many more happy finish line celebrations. 

Sounds like you are a natural!  Great job and good luck!