Part of me thinks it would be easier for my family if I told them in advance, helped them to understand my position. In reality, it's not so simple. They likely will have you institutionalised, which to me would be like hell. But I suppose it depends on your family. I once saw a news video about a guy who had awful mental health issues which resulted in conversion disorder as well. His parents supported his wanting to die and tried to push for legislation to pass. It took too long, the guy died alone in his hotel room.
Depending on your relationship with them, leave them a note. Tell them it is not their fault (if that's the case). Tell them everything you need to, and everything you can to lessen the blow. I won't tell a soul in my life. No one would understand. I will leave a note, and a video. I will die alone, just as most people do, CTB or not.