There is a fan-fic where Olivia takes Peter to see Rachel and her husband and of course has to tell her sister that she is pregnant by a man she has only known for a few months. She pretends that he has been working under-cover with her for a few years and she couldn't tell Rachel about him because of the job. It's in the Fringe fan-fic archive but I can't remember which category or date it was written.
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