Grad party hostess confronts plus-one guest who didn’t introduce herself, guest leaves the party to sit in the car alone: “I didn’t know whose house it was.” – FAIL Blog

This person thought that kind of everyone was to blame. If anything, I’m surprised that the BF’s Mom didn’t introduce them, since the hostess was her sister. The BF might’ve just not known the etiquette of introducing his GF to his aunt, or perhaps it slipped his mind due to the large amount of people at the gathering. 

Plus, the party was for a cousin, not the Aunt, so it makes more sense to find the cousin and congratulate him, rather than automatically seek out the aunt.