My boyfriend [30/M] of 4 years calls me obsessive when I [23/F] am upset that he constantly changes plans

My boyfriend is a very busy person who changes our plans often, and usually last minute. Whenever I get upset at his flakiness, he usually gets mad at me at first for not being "understanding" enough and being too "obsessive" about our plans, but then apologizes some time after. This time he's trying to reschedule our small thanksgiving get-together. Because I know of his difficulty in keeping plans, I decided to have him choose the date that was best for him more than a month in advance. I then told this date to our friends and they agreed.

This morning he called me to say that his sister is suddenly coming into town that specific weekend and they will go on a family trip. It wasn't so much the fact that he decided to change plans (again) that upset me, but the way he said it: "My sister's coming into town. Ask (our friend) if we can do another week. If not, go without me. Thanks for understanding." He didn't feel the need to apologize, or acknowledge the fact that he was troubling both me and our friends, or acknowledge the fact that I set up this whole thanksgiving to begin with because he said he wanted to meet with those friends for thanksgiving several times. Now I will probably have to go alone.

I know I could ask our friends to reschedule, and there's a possibility that they can, but I feel embarrassed to do so because they already bought all the ingredients and were expecting us on that date. One of them works on the weekends and would probably have to re-adjust her work schedule. My boyfriend doesn't want to ask them himself since I am closer to them, but that hardly seems fair because it's not me who suddenly decided to choose a new date after choosing the initial date to begin with. I'd be really sorry to my friends and I am angry at his lack of tact.

I know I am being petty for refusing to ask them to reschedule for him, but I am tired of having to bend over backwards to accommodate him time. From his point of view, I shouldn't be disappointed because the situation is out of his control and I am being insulting to him by not understanding the fact that he wants to go on vacation with his sister who he rarely sees. I wish I were allowed to be disappointed without him assuming that I am rejecting his excuse.

I explained this all to him, and he apologized like he has many times before, but I don't think he will end up asking our friends to reschedule. Should I ask for him, or should I just go alone? How can I avoid this type of situation in the future?

tl;dr: I scheduled a get-together at my boyfriend's request on the day he wanted, now he wants me to ask our friends to change plans, and I feel angry and disappointed at his constant lack of follow-through and responsibility.