لَيْسَ لِأَخِي زَيْدٍ أَخٌ
Zayd’s brother has no brother.
The expression illustrates how “Zayd has no brother” may be conveyed indirectly. Assume Zayd had a brother. Zayd would then be a brother of that very person, since brotherhood is reciprocal. The sentence, however, denies that the supposed brother has any brother at all. The original assumption therefore fails.
The necessary consequence of someone being Zayd’s brother is that this person has a brother—Zayd. Negating that consequence refutes the supposition that Zayd has a brother. This helps explain the alternative reading of “laysa ka-mithlihi shayʾ”; the subjects differ, but the inferential pattern is similar.