सङ्करो नरकायैव कुलघ्नानां कुलस्य च।पतन्ति पितरो ह्येषां लुप्तपिण्डोदकक्रियाः॥
saṅkaro narakāyaiva kula-ghnānāṃ kulasya capatanti pitaro hy eṣāṃ lupta-piṇḍodaka-kriyāḥ
Word by word
The verse says
That mixing brings only hell for the family and for those who destroy it. Their ancestors fall, deprived of their offerings of rice and water.
In plain words
The argument reaches the dead.
In this picture the ancestors depend on the living. They are sustained by the offerings their descendants make, and if the family line breaks, the offerings stop and the ancestors fall.
So Arjuna is now arguing that the battle will harm people who died generations ago.
Where you've felt this · in growing older
Nobody has opened the old family house in four years and the garden has gone to seed. Whether it matters to the people who built it is not a question you can settle. It matters to your mother standing at the gate, and that turns out to be reason enough.
Going deeper
The practice behind this verse is real and still observed: śrāddha, the offering of rice balls and water to the ancestors, performed by male descendants.
In that world the family is not only the people currently alive. It is a chain running back through the dead and forward through the unborn, and each generation owes maintenance in both directions. Break the chain and the obligations fail in every direction at once.
Seen that way, Arjuna's horror makes sense on its own terms. He is not only counting the men who will die today. He is counting the ancestors who lose their support and the descendants who never arrive.
But notice what this does for him. He has now widened the damage until it is infinite, which makes the action unthinkable and therefore settles the question. That is a very effective way to stay seated. And it is the second time in this chapter that he has built something enormous out of a body that simply could not hold a bow.