कथं न ज्ञेयमस्माभिः पापादस्मान्निवर्तितुम्। कुलक्षयकृतं दोषं प्रपश्यद्भिर्जनार्दन ॥

kathaṁ na jñeyam asmābhiḥ pāpād asmān nivartitum kula-kṣhaya-kṛitaṁ doṣhaṁ prapaśhyadbhir janārdana

Why should not we learn to turn away from such a sin, O Janârdana, who see the evils in the destruction of a family?

In plain words: Arjuna asks why he and his brothers, who can see how wrong it is to destroy a family, should not step away from such a sin.