PIR Guest Limits Dilema

Good afternoon everyone. I've been hit with a bit of a dilema. My son's graduation is slated for Dec 5, however the number of guests was reduced from 4 to 3 people. When we received our son's latest form letter, we were informed of the change, and he had removed his little sister's name from the list, leaving his "on-again" girlfriend at the top of the list, and my wife and I as his guests. He says he wants to get engaged to her after graduation. I'm pretty sure they won't end up together long term, but of course my wife tells me we have know way of knowing that.

To be honest, I'm a bit upset that he would drop his sister like that, knowing that if we come up there, she will be with us. Unfortunately, she overheard us reading the letter, and she broke down in tears. I'd like to write him and tell him to drop the girlfriend and put his sister back on the list, but of course my wife doesn't want me to start anything. She feels if I act like I don't like the girl in any form, he won't come around very often in the future.

If he is limited to only 3 guests, can he change those names at this time? Should I ask him to remove the girlfriend? If not, As much as I would love to watch the ceremony, I don't want my daughter's heart broken by her own brother, so I will probably have him swap my name with my daughter's. Is there a place I can wait for them during the ceremony?

If it can't be changed, and we come up there with my daughter, will they let her in since she is a minor but not on the list? Will we be asked to leave and be forced to wait until the ceremony is over to see him? Can I substitute her for me, so that she is with my wife during that time.

Just looking for what to expect and/or advice, so I can be prepared.