VSM, I can understand not wanting to have a heart to heart with anyone right now. But your sister and your parents need to know why you're separating (your parents need to know that you are separating) and as time goes on, the real reason will come out whether you tell them or not. When that happens, they'll be devastated and confused about the reasons you haven't told them. And, most likely, very hurt that you felt you couldn't go to them.
Can you bring yourself to take up 2Packets offer of telling your sister? If you were my sister, I'd want to know right away and wouldn't mind at all if your friend rang me. It doesn't matter if she doesn't know 2Pax. Thinking about it, this has some advantages over you telling her. It will give her time to digest what's happening and think about how to approach it and 2Packets will be able to answer any questions she has about ways to help.
I know this will sound completely unappealing and you might think it's a cop-out and that you should be the one to tell them. I have to disagree and think it's more important that your sister, at least, knows now. All you will need to do is ring 2Pax and say "Tell my sister. Here's her name. Here's her number." Then get a bottle of wine, sit yourself down in front of the computer and chat away. You will be scared shitless, I know, but then it will be over and done with. You deserve people to support you and they can't if you don't let them know what's happening.
I wish I had something clever to say about feeling ashamed. You have absolutely nothing to be ashamed of and I wish I could magic it away for you.
Please call -- just get it over and done with. If your friend is not there, leave a message with your sister's name and phone number.
Thinking of you.