Hang on, she still has two spare tickets of her own: "mother and father...my younger sister, and three grandparents. This leaves me two tickets." Who else is she going to invite, other than the several-years wife of her father? I don't know whether she needs a ticket for herself, but perhaps she's objecting to inviting her stepmother because she wants to invite the hot new guy she spotted in the supermarket last week...
Am I missing something here?
One more thing, when I graduated from University, I was allowed two tickets. No space for my sister or fiance, let alone extended family.