It might have to do with perception. A grandchild sounds far removed, and therefore might not seem as appealing as a potential love interest or sexual partner as might a sibling, even if technically either option would be the same age.Interesting. In the previous poll, Daniel was far ahead of Tara's father as the preferred babydaddy; but right now having the baby be her sibling (summed up) has almost double the votes as it being her grandchild (summed up). It could be a time-of-day/timezone issue (the poll hasn't be up for long), or maybe we see what kind of second choice the puppy lovers preferred.
As for me, my priority for impreg was dog>father>son (not a fan of shota lol), and so I voted for a sister-daughter (because more women means more potential for pregnancy).