= Solution
Fix one particular ancestral <allele> copy $q$ among the four copies in the founding couple. The <pedigree founder> carrying it transmits it independently to each of the two generation-2 siblings with <probability> $1/2$. Therefore
$$
\boxed{P(q\text{ reaches both siblings})=(1/2)^2=1/4.}
$$
Here “one copy” means a specified copy, as required to follow “this ancestral copy” in the subsequent parts. If instead one asks whether the siblings share at least one of the four copies without specifying which, the <probability> is $1-(1/2)^2=3/4$: they fail to share either the paternal or maternal copy with <probability> $1/4$. These are different events.
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