I.G.
I am sorry that you have hurt feelings. But SERIOUSLY there is about a million reasons you should get over yourself on this one:
First of all she's a teenager - of course she's not terribly well mannered and occasionally thoughtless - I'd be surprised if she wasn't!
There is a possibility she didn't snub you, maybe that information got lost somehow or the person who was announcing forgot to announce your name.
Even if you were not included by her. Sorry, but this was HER special day and not about YOU! She gets to include whomever she feels is important in her life. Having a decent relationship, does not mean that she feels that you are all that important to her. Yep, that hurts, especially if you have invested emotions in her that aren't reciprocated - but you can't force someone to love you!
Last of all I am pretty sure that everyone was so preoccupied with their own family that nobody thought "she must really hate her stepmom". Thinking that way just escalates the situation in your head.
So again, sorry you were hurt, but maybe this is not something you should make a big stink about, if you want to keep on with your decent relationship.
Good luck!