Sāṅkhya Yoga · Verse 30
"देही नित्यमवध्योऽयं देहे सर्वस्य भारत | तस्मात्सर्वाणि भूतानि न त्वं शोचितुमर्हसि"
"dehī nityam avadhyo 'yaṃ dehe sarvasya bhārata | tasmāt sarvāṇi bhūtāni na tvaṃ śocitum arhasi"
The one who possesses a body is eternally unslayable in the bodies of all, oh Bhārata. Therefore, you should not grieve for any being
The argument universalizes: it is not just the ātman of Arjuna or his relatives, but sarvasya (of all) the dehe (in the bodies). The dehī (possessor of body) is one in all.
Avadhyaḥ (unslayable, from vadha — slaying + a- negative) — the animating principle cannot be destroyed. Nityam (eternally) — not a temporary condition but absolute.
Therefore (tasmāt), there is no object of grief (sarvāṇi bhūtāni — all beings). Arjuna should act without attachment to result because there is no real result of death.