When is family not family?

I just got an invitation to a party. An actual printed, US-posted missive asking me to join in a celebration. How cool is that? I love parties!
And I plan to completely ignore it.
Before you ask why I've got a stick up my butt about this particular party, let me explain a few things:
I'm the youngest of a LARGE family. My eldest sibling was in Vietnam when I was in grammar school, so the age gap is pretty significant. Most of my brothers and sisters are/were married and have kids, so the extended family is the size of suburban Chicago.
To illustrate this disparity in ages, let me point out that I have three grand-nephews (is that a word? sons of nephews?) and a nephew-by-marriage (my brother's stepson) also has a full family of his own.
And then there are the cousins. My family's fertility apparently knows no bounds.
I have...wait, let me count them up...I have to get a piece of paper and list them. Okay, how horrible is it that I had to go and find the Excel spreadsheet that a distant cousin made up during a family reunion several years ago?!?! I couldn't remember them all!
When I was a little kid (attending one of the aforementioned family reunions), I thought an elder cousin was, in fact, my aunt. Honest mistake, though, since there is just three tiny years' difference between me and that cousin's oldest CHILD!
Okay, here's the total. I have 20 first cousins. Add to that eight siblings, those siblings' kids (18 plus 1 stepson) and you have an army that could take over Australia.
Back to the invitation.
It's not from one of my brothers or sisters.
It's not from a nephew or niece.
It's not from a cousin.
It's an invitation to the eighth grade graduation of the oldest son of a cousin that I see approximately twice per year.
WTF?!
Just because we happen to have some watered-down version of the same blood flowing through our veins is not good enough reason to invite me to your party. (I know the invite was his mother's idea, not his. Still.) This teenager and I would not recognize each other if we passed in the street.
I'm not going to the party and you can't make me.
Where do you draw the line?