I'm expecting my fiance to invite me to her boss' party.
Has she invited you to any of her work events in the past?
She has not invited me to one yet and I don't think that's fair to me.
Wanting to be part of your fiance's work life is normal, but sometimes it is not possible depending on the job. It would not be good to assume that you can go to a boss' party when she might not be able to invite you. Then you will be disappointed and so will she that you are.
I was going to cook for the party too.
You intentions are undoubtedly good and it's kind of you for wanting to cook for the party, but the expectation to be invited is a bit unreasonable. Could you ask her if the event is appropriate for her to invite you? I'm sure she would if she could.