Kamsa addressed his sister and brother-in-law (Devaki and Vasudeva) as maha-bhagau because although he killed their ordinary children, the goddess Durga took birth from them

Expressions researched:
"Kaṁsa addressed his sister and brother-in-law as mahā-bhāgau because although he killed their ordinary children, the goddess Durgā took birth from them"

Srimad-Bhagavatam

SB Canto 10.1 to 10.13

Kaṁsa addressed his sister and brother-in-law as mahā-bhāgau because although he killed their ordinary children, the goddess Durgā took birth from them. Because Devakī bore Durgādevī in her womb, Kaṁsa praised both Devakī and her husband. Asuras are very devoted to the goddess Durgā, Kālī and so forth. Kaṁsa, therefore, truly astonished, appreciated the exalted position of his sister and brother-in-law.

SB 10.4.18, Translation and Purport

O great souls, your children have suffered their own misfortune. Therefore, please do not lament for them. All living entities are under the control of the Supreme, and they cannot always live together.

Kaṁsa addressed his sister and brother-in-law as mahā-bhāgau because although he killed their ordinary children, the goddess Durgā took birth from them. Because Devakī bore Durgādevī in her womb, Kaṁsa praised both Devakī and her husband. Asuras are very devoted to the goddess Durgā, Kālī and so forth. Kaṁsa, therefore, truly astonished, appreciated the exalted position of his sister and brother-in-law. Durgā is certainly not under the laws of nature, because she herself is the controller of the laws of nature. Ordinary living beings, however, are controlled by these laws (prakṛteḥ kriyamāṇāni guṇaiḥ karmāṇi sarvaśaḥ (BG 3.27)). Consequently, none of us are allowed to live together for any long period. By speaking in this way, Kaṁsa tried to pacify his sister and brother-in-law.