If it’s clear that it’s an occasional three-line whip family event then I do not think you are unreasonable at all to expect your dh to attend op, especially if it’s something that is important to you and your parents. He should want to support you. It was a bit immature of him to leap on to the bandwagon of your brother’s gf flaking out!
It’s an unfashionable view on Mumsnet, but part of marrying a person, imho, is showing support for your spouse’s family, except in the case of unreasonable or abusive behaviour of course, or at least wanting to support your spouse in supporting them!
The real issue here is that your dh has a different view of what constitutes family. Can you talk to him seriously about it when you are back at home? You are allowed to have differences in what is important to you both, but you both need to talk about it openly and both make compromises.
Maybe you could drop the expectation that your dh accompanies you on all the frequent visits to your family op, in return for the commitment that he shows up fully on three-line whip occasions like this charity event?
And equally, if he expects you to do something for him that you find tedious, not necessarily related to family visits but it could be, then you both need to compromise on that too.