Why is your mother deciding who goes to your graduation? Do you want your SO there over this family member? Then you invite your SO and tell that family member you’ll see them at the house.

It’s YOUR DAY, not your mother’s, not your gf’s. I’m more concerned that your mom just steamrolled your ticket choice. It seems like you picked your SO and your mom said “no! Pick Family!”

Your SO is also 16....sorry but at 16 I didn’t react perfectly to moments where I had high levels of emotions. It can hurt wanting to be involved and being excluded instead, there are adults who still can’t act appropriately. She’s human and tell her she’s being manipulative whether she intends it or not.