I was in a very similar spot with my own Graduation. I got only 6 tickets and way to many people that wanted to attend. So when time came for me to give out my tickets I picked My Mom, 2 sis, bro, grandmother, and god-mother. My mom remarried 2 1/2 years before (my father passed away when I was 9) And as much as I like my stepfather I wanted to pick the above to attend. Most people told me to give my stepfather the ticket and not my god-mother. But, my god-mother helped raised me after my father died she became my 2nd mom over night and she never once complained. She did more for me growing up than she had to.

Maybe aunt and uncle in the OP are the same way we don't see how close the grad is to their aunt and uncle. The grads aunt and uncle might live next door and helped to raise them after mom and dad split. The grads aunt and uncle then become like 2nd parents.

My advice to the grad is set down with dad and tell him why you want to invite your aunt and uncle over your step mom. Give him all the reasons. Let him know that you do not want to leave his new wife out but that with the limited number of tickets you had to make the tough decision. Let him know why you made that decision. If you made this choice just because your moms family doesn't like or accept your dads new wife then you need to set down and look into yourself and make the decision for you and not your family. If you made the decision because the of a long close relationship with your aunt and uncle then you need to stick to that decision.