As your younger sister, this poor girl Devakī is like your own daughter and deserves to be affectionately maintained. You are merciful, and therefore you should not kill her. Indeed, she deserves your affection.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
एषा तवानुजा बाला कृपणा पुत्रिकोपमा । हन्तुं नार्हसि कल्याणीमिमां त्वं दीनवत्सलः
eṣā tavānujā bālā kṛpaṇā putrikopamā | hantuṃ nārhasi kalyāṇīmimāṃ tvaṃ dīnavatsalaḥ
SB 10.1.45