Sri Aurobindo himself said that it is The Mother’s coming that made it possible to fulfil the Yoga. Before the Mother’s coming he could realise the Supramental consciousness himself but earth consciousness and humanity would not be able to follow it. It is The Mother’s coming that made it possible.
As to outer relation, again in the original story itself Savitri is regarded always as the daughter of the Sun that came to Aswapati’s house as a result of the boon. Though the outer relation is of father and daughter, the inner relation is of the Yogi Aswapati and his Shakti that was born to him. We see this in the story of Brahma and Saraswati ji as well. She has come out of Brahma so is his daughter but she is also his power without which Brahma ji could not create. Hence he takes her also as his consort. Similarly Sri Aurobindo called the Mother as Mother because She is the eternal womb of all creatures and the Ishwara himself is formed out of Her. But equally She is the fulfilling power of Sri Aurobindo the yogi, his Divine consort so to say. Hence the Mother would address him as the Lord.
Finally, as Sri Aurobindo himself said, these are symbolic in nature and ‘have nothing to do with his present life’.
Affectionately,
Alok Da
