Hiya,
First off I do realise this post may / will come off very harsh. It's not meant that way, I'm just trying to show you you can see it differently too.
If I only told my mum and told her not to tell anyone else my sisters (I too have a lot (4 of em)) would find out eventually.
Ignoring the fact that mum told them against your wishes, to prove my point, why shouldn't they know? I'm unsure of how close you guys are, obviously, but my sisters have all been very supportive of the diet. Worried, yes, but supportive! I really think you'll need that support hon.. try explaining to them how the diet works and show them the booklets. I explained to my family, in tears I may add, how terribly unhappy I was about my weight and even my cynical dad took this diet seriously from then on.
As for the rest of the world knowing, through them, yea that sucks. My sisters have kept it to themselves, mostly, and not told family because I begged them not to. They have however told their friends but looking back... me getting pissed about that was kind of unjustified anyway. They are probably just really worried and need someone to talk to about your diet. I can understand how you think it's none of their business and as said before I don't know how close you are but I couldn't have done this diet without the support of my family and sisters.
You may need their support sooner than you think as this diet is pretty rough at times.
Try explaining to them how it all works, they know you're doing it now so might as well explain it right? Try looking at it from a different perspective, this perspective will probably make you very upset and angry for quite some time to come
Explain to them all, (= including sisters) why you want to keep it low key and that you feel it's your journey and people shouldn't interfere etc. It's better to explain all that now so they will respect your wishes.
You could talk to your mother and tell her how unhappy you are about her telling others. Did you specifically tell her not to tell anyone though? Like, explicitly without flowers 'dnd pink cloud-talk around it? After she'd told 1 person, that 1 person could've been the one to tell all the rest. If she didn't know you wanted to keep it that private she didn't know she wasn't allowed to tell so... then it would seem a bit unjustified to be so upset with her, don't you think? If she did know then yes, obviously, that sucks and you should have a serious talk with her about it.
Hope it makes sense. I've had pretty pissy moments at my family, too, but in the end they will probably support you if they understand your reasons so best to grab that support with both hands now they know.
xxx