مِثْلُهَا تُخْرِجُ النَّصِيحَةَ لِلْقَوْمِ فَلَاةٌ مِنْ دُونِهَا أَفْلَاءُ
Meaning: Such kinship brings forth sincere counsel for the people; without it stretches the barrenness of distance and separation.
Mithluhā points back to the kinship described in verse 63. Al-naṣīḥah is sincere counsel rather than convenient flattery; falāh ... aflāʾ supplies a picture of distance and emptiness.
Kinship is personified as a source from which counsel can be drawn, while the absence of that relationship is conveyed through the language of desolation. This is an argument offered by the poet, not a universally valid rule about every relative.