Its not so much the age Cam but as Tamba said more the thought of your mum still 'doing the deed' if it was a stranger that was 40+ and pregnant then it wouldn't be strange in the slightest. (btw my mum isn't prgnt im just speculating) 
LOL@ Tamba about your 'sex year old brother'... I can imagine thought that when the age gap is that large the relationship is more like niece/nephew, I think that I would be in danger of being to motherly toward my little bro/sis if there was that much of an age gap....how old did you think I was?? (out of interest)