Wiktionary
be mother
vb. (context idiomatic British English) To pour out tea for others.
Usage examples of "be mother".
That much she had known from the moment he had been given into her hands at his Naming, she to be mother and father to him both, for his Lady Mother was untimely slain in the Chaos Lands defending their realm, and the Lord Prince his father had no time for mewling infants.
Cook might not be mother to the world but she was queen of the Stantnor household.