Ptolemy assigns the mother the Moon and Venus in the same chapter that gives the father the Sun and Saturn, and the symmetry is exact. Each parent receives the luminary of one sect and one further planet of that sect. The Moon and Venus are the night team's light and its benefic.
What the Moon contributes is bodily: nourishment, the rhythm of feeding and growth, the first months of a life, the condition of the body the native was carried in. What Venus contributes is relational: how the mother was regarded, whether she was loved and by whom, the presence or absence of ease around her. A tradition that wanted a single mother significator would have chosen the Moon. The pairing with Venus is what lets a reading separate the mother's circumstances from the native's experience of being mothered.
The condition the astrologer checks is the same in both cases. Is the significator in a sign it likes. Is it aspected by a benefic or besieged between malefics. Is it in an angle, where it acts strongly, or in a cadent place, where it declines. Ptolemy reads the parent's fortune, rank and length of life from those questions, and he is careful that the chart describes the parent as the native encounters them, which is not the same thing as the parent's own nativity.
Neither Ptolemy nor Valens gives the mother a house of her own. That arrives later.