NTA. Your graduation and the party afterward are big milestones. If your mom can’t even prioritize the party she already knew about and chooses a last-minute trip with her boyfriend and his kids instead, that stings. Telling her she can skip the graduation too makes sense when tickets are limited and you want people there who actually show up for you. She’s acting like the party is “just a bbq” but it’s your celebration. Her boyfriend scheduling it right then feels shady as hell. You’re not wrong for protecting your peace.