How do you un-do a serious screw up!?

I just screwed up big time! I had with my sister-in-law (brother's wife) a few months ago and didn't use any protection. We both thought it was pretty dumb and put it in the past without a word to anyone. Long story short, now she's pregnant! My brother is stoked cause he thinks it's his - she went to the doctor and found out it's mine! What should I do. I can't be with her - we're not that close. I can't tell my bro or he'll kill me. She doesn't want to have an abortion, but let's face it - as soon as the kid is born my brother is going to know it's not mine! What do you all think I should do.... I'm at a total loss except for running to Mexico and dropping off the face of the earth!