What should I do?

So I've been studying abroad this semester, and my SO and I had agreed that we would go see the new Star Wars film together. We made these plans like before I even left in September.

Well, he's a twin and for their birthday his family wants to go do dinner, and then go see Star Wars together. Kevin, my SO, told them he would do dinner but not the movie because we had plans to see it together, and we have had these plans for months.

Well, his twin brother has been a dick about it. He's always been really mean to me. I think it's just because he thinks I'm taking too much of Kevin's time, but I mean I am his girlfriend of nearly four years. I don't think I take up too much time and also I've been away for four months now.

I would feel like a bad girlfriend if it was my fault that he didn't go have this day with his family. However, I also feel disrespected that these plans were made in spite of our plans.

I have ultimately stayed out of it, & Kevin hasn't made any changes to our plans. I feel like he won't go with them unless I verbally tell him it's okay.

What should I do? Stay out of it? Tell him to go with his family? (His birthday is right before I come home, the 16th and I come home the 22nd)