I came out to my immediate family as nonbinary partial transmale with a brief, casual, group email that explained my feelings since childhood, gave definitions, and links for further information.
This avoided putting them on the spot, avoided their forgetting or confusing what I said- since it was in writing, and avoided forcing a confrontation. Since it was a group email, nobody felt obliged to respond, so it was mostly ignored, to my relief.
But now they know. They could believe or accept it, or not, but at least they didn't have to do it on the spot.