Surprised there aren't more examples, as I've seen a number of these. At least two where Nodoka was unknowingly preganant when Ranma and Genma left, another where Ranma's sister was taken or something, and Nodoka had hoped 'Ranko' was her when she first came to the dojo in canon.
Of course as usual I can't remember any of em....
There's also plenty where the sibling gets born sometime post canon...






