Full Idea
There may be a regularity of siblings looking similar, but the tie that binds them is not their similarity, but rather their being born of the same parents.
Clarification
'Siblings' are brothers and sisters
Gist of Idea
Similar appearance of siblings is a regularity, but shared parents is what links them
Source
Alexander Bird (Philosophy of Science [1998], Ch.1)
Book Reference
Bird,Alexander: 'Philosophy of Science' [UCL Press 2000], p.49
A Reaction
A nice objection to the regularity view. Regularities, as so often in philosophy (e.g. Idea 1364), may be the evidence or test for a law, rather than the law itself, which requires causal mechanisms, ultimately based (I think) in essences.
Related Idea
Idea 1364 Locke confuses the test for personal identity with the thing itself [Reid on Locke]