It's his kid too. If you were close enough to sleep together, then you should have been close enough to tell him. I'm not saying you should have asked his advice or let his opinion factor in to what you are going to do with the pregnancy (ie abort, have the child etc.) but he deserves to know...

Doesn't matter that you're not together. Makes no nevermind that it was a one-night stand. Totally irrelevant that you barely know each other. He deserves to know. Like Gothtart says, it is still your body and your choice whether or not to have/keep the baby, but you are obliged to tell him that his arrow hit the mark. And if you do decide to have and keep the child, he is obliged to help you with support, even if it's not actual support but only financial...

I don't think you're wrong. It was not your sister's place to tell him. If you felt that it was best to not say anything, then she should have respected that decision. As long as you knew for sure that you would never ask him for anything with regards to your child, there's no reason he should know. You were both willing to proceed in having a one night stand without protection so....you are the one that has to really deal with the consequences of that decision. Although telling him would probably be the best thing to do, it's not necessary if you're ready to face it alone...