We're having an informal wedding, and we want our invites to reflect that so people know what to expect. It's outdoors, essentially camping, and for people who ask us what to wear the answer is basically "it's outdoors, in the evening, in the mountains, in May. Dress comfortably and be aware of the weather. Jeans and sweaters are perfectly fine."
My question is, as we address the envelopes on the invitations, do we have to use titles? It feels very odd to call my brothers "Mr. X" and also seems like it's giving an inaccurate representation of the formality of the wedding. If we're writing the envelopes to match the formality of the wedding, it seems like we shouldn't use titles. However, do we need to anyway for etiquette reasons?