I can understand the hesitation with sharing. I didn't want to tell my mom or sister for many of the reasons you all listed but when I did tell them, I was TOTALLY surprised at their reactions. They are way more supportive than I could have imagined. Of course it is your decision whether or not to disclose but I've adopted the mentality that I am doing this for me. Who cares what they say? I know it's easier said than done but you really have to think about what matters to you. You are your first priority and have no reason to feel ashamed for taking control of your life. You said it yourself, they have no idea what it is like to be 100 lbs overweight so there is no way they could ever relate. Do you plan on ever telling them or will you keep it to yourself forever?
Also, I have 2 sons ages 7 and 10 and I decided to tell them and I made sure to explain what was going on and why and to answered all of their questions. I even showed them one of the videos from www.weightlosssurgerychannel.com Check it out if you haven't already! Very informative,