Am I silly to feel this way or are my feelings understandable?

I think both you and your sister are acting as would be expected under the cirstances. If, as you say, you've never had with this girl, then you've got nothing to worry about. Bide your time until the bub arrives, get a DNA test, accept everyone's apologies graciously, come out of the whole revolting mess the bigger man!! You know you're gonna win. Just sit tight mate. Your moment of truth is getting closer with everyday that the pregnancy progresses. See you on Maury! "You are not the babies father!!!!" ha ha