So this March my grandparents on my dad's side of the family will be having an anniversary meal and on the list of those invited are: me, my mum, my dad, my sister, my aunt, my uncle, my cousin and his gf, my other cousin and her bf, my other aunt, my other uncle, and their daughter (my third cousin) and her bf. 

Now normally this would look quite normal were it not for the fact that apparently my gf can't come because

1)my sister would be "left out"
2)we've not been going out that long

Now in response I have the following things to say

1)my sister likes my gf and vice versa, they get along really well
2)by March me and my gf will have been going out for 7 months. 

My mum and my sister both think it's utter bullshit for my cousin's to have their partners but not me. 

Personally I'm appalled and more than a little resentful over this, but am I the asshole?

Now I love my grandparents to bits and I love my uncle and his family (aunt and two cousins) to bits, they're some of my favourite family members but I think it's quite rude and more than a little disrespectful to not invite my partner when everyone else has theirs. 

Even my sister leapt to my defense saying "how will randompasserby feel then?" When hearing about the exclusion of my gf. 

(For the record I'm not fond of my other uncle and aunt mainly because with her *everything* **has** to be a fucking competition from height and age to achievement and education, they look down on everyone who isn't like them and honestly I'm not fond of them, because they can be exceptionally rude)

But again, am I the asshole for being angry at them for excluding my partner even though it's their event and they choose who can come?

Oh, I learned this today btw, and I haven't said anything to anyone but my mum on this issue, I don't want to stir the shit pot any more than necessary. 