I do have a trans family member, but I don't know them very well.
Anyway, I thought my mother would understand what I was talking about when I came out, because I recalled some conversations from my teens with her where she likened herself to a man without a penis, or a woman with a man's personality, or something like that. It was a rather casual conversation though, as many of our conversations about potentially deep subject matter were. But it did stick in my mind.
However, it turned out that she did not understand the reasoning behind transition at all, and declared that if she had never felt the need to change gender and undergo operations etc., she did not understand why I would. So it was just me assuming she understood because of an off-hand comment she once made. It seems that, in the end, her comments about being a man in a woman's body were nothing like the experience of being an FTM. She doesn't understand what I've been through at all, and after a few conversations since I'm fairly sure she has no idea who I am or what I'm like any more too.