Bette 22, I understand and sympathise with you for feeling a bit miffed, but as you say, you will attend your niece's wedding and will hide your feelings. It is the bride's big day and none would want to spoil it.
Unfortunately, weddings and funerals are highly emotive occasions, and we can be sensitive about our status on that occasion.
Personally, I've never liked the idea of "the main wedding" and " the evening reception". It feels like guests are being divided into two classes. On two occasions, we experienced the invitation to the evening party, one was my niece and we realised that all the bride's relatives had this invitation, while the groom's were at the main reception. Needless to say, we did not show our feelings but it did seem a bit odd. The second one was the daughter of my DH's friend, the only one present at the wedding.
Although DH was very hurt and upset re the evening invitation, we managed to hide our feelings, saying again it's the bride's day and nothing can be more important.
I'm sure you'll attend and have a great day in spite of your misgivings.
We cannot control our feelings, but we can control how we react to or handle them.