Is it normal for a grandmother to miss her grand daughters 21st b'day?

Before you answer, allow me to give you a basic understanding of my Grandmother (Nana, as we call her)...
My Nana is my fathers mother. She has two other children, and for as long as I can remember she's clearly favoured her other children over my father. My dad knows this, as does my mother. We're not sure why. My father has always been very respectful of his mother, and very loving towards her. His two siblings were always more rebellious. They've caused their fare share of family drama's and given their mother grief. I'm not saying that they're bad children to their mother, or bad people. I'm merely stating that my father is no lesser of a good son than his siblings, and has done nothing to earn his mothers disdain. Despite this, my Nana clearly favours the other two children. Because of this, she favours the other grandchildren over my brother and I. There is a very distinct family hierarchy, and my brother and I are at the very bottom (just above my mother). It's always been this way. For example; my Nana gave my brother and I $50 for our 18th birthday. She gave my cousins $100, and gave two of the most favoured ones $150. We didn't complain. I was rather happy with the $50.
My Nana also spends her weekends and most weekdays with her other grandchildren. She goes on holidays with them (that we are specifically not invited to), she organises outings with them. She loves them more. It's obvious. She favours her 4 other grandchildren more than my brother and I.

Now, let me give you a basic understanding of my brother and I...
We are good kids. We have never gotten in any trouble with the police. Never taken drugs. Never gotten pregnant (or gotten anyone else pregnant). We are the only 2 grandchildren to finish high school, and I am the only grandchild to have gone to college. Those are the kinds of things that most grandparents would be proud of, isn't it?
I buy my Nana gifts for every occasion (she ASKS me to, weeks in advance), I pop over frequently, I listen to her complain about every single person on the earth (including her beloved grandkids). I put in a damn effort.

So that's us. We're a messy extended family, most of which have deep-seeded issues, but we're still pretty close.

Now, the fight.
My father and Nana got into a verbal argument about a month ago. I won't go into details, because I've already been blabbering on for long enough. All you have to know is that the fight itself wasn't too serious. It started with a Facebook comment I made to a friend about disliking a certain actor. My cousin saw this and, for some reason, told my Nana. My Nana got offended (she likes this certain actor) and told me never to say that again. I was a little blown away by this, but I agreed. No foul language was used at any stage. My Nana then called my dad telling him what I'd said. My dad (who I'd told. We laughed about it) told my Nana to stop causing a fuss and trying to "stir the pot". The fight escalated from there. It never got really nasty, but it did end with my dad hanging up on her. Since then we haven't heard from any member of our extended family.

My 21st birthday is coming up in a week. My dad's 50th birthday is a day before mine. Normally the family come over and we all have a laugh. Considering the circumstances, and the fact that my Nana hasn't spoken to us, I am starting to doubt that their presence will be made. My Nana has never loved me as much as other members of our family, but I never thought that she's miss my 21st because of some silly argument. I know that my close family (my mum, dad and I) will have fun regardless, but I know that I'll feel a little miserable and depressed if my Nana doesn't show... which I doubt she will.

So, is this normal? Do grandparents frequently miss their grandchildrens birthdays out of spite?