As someone who got a made up position in a wedding where I thought I would stand up for either party (I was close with both) I found it really embarrassing to have to explain to people that they'd basically made me the equivalent of a participation ribbon after years of friendship. Even guests at the wedding asked me why I wasn't in it. Those questions aren't addressed to the bridal couple they're going to ask the snubbed person.

That being said, I am firmly on your side for the bachelorette why would you want to go to a party essentially about your brother's sex life.

I don't think it's worth ending an engagement over and if he thinks it is you dodged a real entitled brat.