Brother's wedding reception

My Brother got married last fall, and I went to both the wedding and the reception and all the events that go along with it. Here's my problem: They are having a larger reception this September, but I simply cannot go. I have a partner that I have been with close to 4 years now. She is starting Grad school. Her move in date is the same day as the reception. We are driving across 4 states to move her in. This is a giant life step for her. She is nervous and I feel like I need to be there for her. Not to mention I will not be able to see her until Christmas again. I told my brother about this and he understood, but got a text from his wife saying about how sad he is about this. I get that he's sad, and if it was literally anything else I'd be there. I even tried for an entire week to shift things around, but it's a hard move in date. Am I being a bad person??