[Jtsummers]

Siblings don't share a guaranteed 50% of DNA, there's actually a chance (though remote) that they'd share no DNA.

Assuming each chromosome of a pair has an equal chance of being passed on (someone else got info on what, if anything, is known to change these odds?). So there are 2 x 2^23 possible chromosome sets. While the odds are low, it's entirely possible that two siblings with the same parents would have less than 50% of their DNA in common.

In the case of a chimeric individual, if they already were below 50% DNA in common, then the children they produce might appear as only distantly related, nieces and nephews at best.

[ronaldx]

True in theory, but unlikely to the point of irrelevant.

Assuming chromosomes are inhereted whole, the probability that siblings share less than 25% DNA - the equivalent of one further generation away - is 0.00531 (0.5% chance).

But chromosomes are not inherited whole - they recombine. This hugely reduces the probability even from that start point.