Of Irish-English origin, the phrase (as) cold as a stepmother’s breath, and variants, mean extremely cold:
– literally, i.e., with reference to low temperatures,
and
– figuratively, i.e., with reference to lack of feeling, of emotion.
—Synonym: (as) cold as Pharaoh’s heart.

These are the earliest occurrences of the phrase that I have found, in chronological order:

1-: From an article describing the poorest Irish emigrants leaving the port of Newry for America, published in The Newry Examiner (Newry, County Down, Ireland) of Saturday 12^th April 1834:

Grief they must have; but every manifestation of it is suppressed. Joy they cannot have: they are parting, for ever, from their mother land; and though, to them, it always had the coldness of a stepmother’s breath, their hearts still warm to it.

2-: From The Vindicator (Belfast, County Antrim, Ireland) of Saturday 5^th September 1846:

Were not this so much a subject of sorrow we could not refrain from laughing at the ruin which the Irish landlords have brought upon themselves. They were content that Englishmen should manage the affairs of this country—they displayed, with a narrow-minded vanity, their equipages in the growing towns and fashionable resorts of England, while their properties at home and Irish towns were decreasing in value, and falling into decay—they gloried in British doings, and forgot their native land—they exacted rents, which land, owing to the decrease in its value, consequent upon the general decay of the country, could not yield. Murders and coercion bills followed. Trade and commerce died of the fear these tumults generated. The stepmother breath of England withered up the bloom of the country. In this state is Ireland, when famine comes to complete the wreck.

3-: From the account of a meeting of the shareholders of the Newry and Enniskillen Railway “and others interested in the prosperity of Newry”, held in Newry about the construction of a railway—account published in The Newry Commercial Telegraph (Newry, County Down, Ireland) of Thursday 16^th March 1848:

Mr. C. Denvir […] said that […] the origin of the project is within the memory of all present, and he (Mr. Denvir) would ask, for what purpose did the people of Newry meet and combine together, in 1844, when the ardor and impetuosity of those who projected this line was at the highest? Was it to make Mr. J.’s line from Newry to Portadown, in order to assist in ruining their town? No. It was to make a direct line to Armagh, and for that purpose only. But even then there were enemies in the camp, who attempted to throw cold water on the Armagh line, which was obtained in spite of them. But it was received with a stepmother’s breath; and from that day to the present, the same secret enemies have been at work, laboring to destroy it. Two years and upwards have now elapsed, and yet the line to Armagh appeared almost as distant as ever.

4-: From the account of a police-court case, published in the North British Daily Mail (Glasgow, Lanarkshire, Scotland) of Friday 2^nd February 1849:

An elderly female, named Harvey, with all the external show of widowhood—in those “weeds” of which Hamlet would say, “These, madam, seem”—presented herself in the witness-box, to give evidence against a coarse-looking fellow, her step-son, for breaking open the door of her dwelling-house in Bridge Street, on Wednesday evening, and otherwise conducting himself in a disorderly manner at same time and place.
In the course of her examination, the old woman made out a case against the prisoner, which would likely enough have verified the old proverb, “Cold as a stepmother’s breath,” had not the disreputable prisoner deserved all the wicked things that were said about him.

5-: From The Armagh Guardian (Armagh, County Armagh, Ireland) of Friday 6^th May 1853:

A Good Stepmother.—ln the year 1846 Thomas Prendergast, a poor man, lived in the neighbourhood of Rockcorry, county Monaghan, striving by every possible means to support a wife and three small children. About that time his wife took fever and died, leaving poor Tommy and three children in bad times and poverty. Tommy, however, after a little, began to sing (as I frequently heard him), “a man without a wife is a beggar,” &c., and as might be expected he got a second wife (a stepmother) for his children—a Miss S. C. Richardson, the subject of my story, with whom Tommy and children went to Belfast to seek employment, where Tommy and one of the children died. The poor stepmother, unable to provide for the two remaining children, left them with the grandmother by the maternal line, and started for America, from whence she sent £5 the other day to the stepchildren. I read her affectionate letter to them, and could wish every stepmother’s breath to equally kind and agreeable.—A Correspondent.

6-: From a letter that a person signing themself ‘Rusticus’ wrote about a snow storm near Belfast, published in The Banner of Ulster (Belfast, County Antrim, Ireland) of Tuesday 10^th January 1854:

You gents in town can have no idea of the grand sights that we have in the country just now, if it were not for the terrible drawbacks of stranded carts and the sudden arrest laid on the eggs, butter, and other farm produce, straight on their way to town, and that “looped and windowed raggedness” through which stern winter, with a stepmother’s breath, now finds unobstructed passage.

7-: From Saunders’s News-Letter and Daily Advertiser (Dublin, County Dublin, Ireland) of Friday 20^th October 1854:

For several days past the temperature in our moist northern latitude has been steadily falling; and yesterday, with that biting north easterly breeze which makes free with noses in a most uncourteous style, and is frequently used as a synonym for “a stepmother’s breath,” it had descended to a point which the thermometer rarely indicates at this period of the season, except before the advent of an early and severe winter. The weather, however, cold as it has become, is favourable for the farmer’s field operations; for those of the barn, the more bitter the blast out of doors, the more merrily swings the flail.—Belfast Chronicle.

8-: From The Belfast Daily Mercury (Belfast, County Antrim, Ireland) of Saturday 30^th December 1854:

The Belfast “Government” School of Design has been closed [by] the King Cole of Marlborough-house—he who fiddles away there, as one of the duumviri who compose the Anglo-Russo autocracy of an institution known to the British public and the parliamentary estimates as “The Department of Science and Art.” […]
[…]
These young Schools, the children of a great experiment, needed fostering, sustaining guardianship till they grew into adult strength; but the stepmother breath of Marlborough-House has chilled and lamed their struggling limbs, before they had cast off their swaddling-clothes. Take half of the mother’s milk from her infant, and supply no commensurate nourishment, and the poor helpless child, which might otherwise grow to manhood and greatness, will sicken and die. So with the Belfast School.