Anonymous User wrote:OP here. Thanks for the responses, everyone. I don't want to get into specifics, but I will say it's not a funeral, a sibling's wedding, or anything quite that important. On the other hand, it's not like missing a birthday or family reunion; people are going to be mad and stay mad for a while. But they'll move on eventually.
What I really was struggling with was the importance of going to the event, since I'm new to the biglaw world and unfamiliar with its unwritten rules, norms, etc. It sounds like the consensus is that missing the retreat would look pretty bad.
You're family/friends etc. should get used to you letting them down as you head into biglaw (half joking).
In all seriousness, I think the advice here is spot on; because it's a formal retreat and not just a happy hour etc., I would miss the family event. You won't get no-offered because of it, but you might rub some people the wrong way and it's not the way you want to start off your career with a firm.