I believe I do have a timeline of events that you might find enlightening.
Gally informs her parents, Patrick and Lettie, about the plans that she and Susanna have to tour the country as a musical act.
Her parents are hesitant, but are somewhat reassured when Gally tells them that Susanna has recently married a Captain Noone and that he will be travelling with them.
Gally’s brother, Newt, has been on the sidelines of the discussion. When their parents withdraw to discuss the matter, Newt asks Gally more questions about Susanna’s supposed husband.
Newt deduces that Captain Noone is a ruse. He tells Gally that the play on words with ‘no-one’ is not as subtle as she might think it is.
Gally tells him that it is too late to change the name from Noone, not least because she and Susanna are planning to rename their act ‘Midnight & Noone’.
The two discuss Gally’s new stage name of Midnight, agreeing that it is better suited than Nightingale given Gally’s low voice. This gives Newt an idea for a song in which Gally would play the part of a man who impersonates birds.
Gally warns her brother, Newt, that she has a “super colossal” favour to ask him.
Through cryptic references to Susanna’s pretend husband, Major Noone, Gally asks if Newt would father a child with Susanna, which Gally and Susanna could then raise together.
Newt is reluctant to do so. In his life generally, Newt has no interest in the required activity and never intends to try it. He remarks that there are other ways of achieving the same effect.
At this point, Gally begins to speak more plainly. She had particularly hoped that Newt would do this for them, so that the kid would be the closest possible thing to being Gally’s own child.
This convinces Newt and they agree to the plan. He asks to be part of the child’s life in the role of an uncle.
Gally begins to explain the practicalities of the arrangement by asking if Newt knows Crewe.
While sharing a drink, Vanessa asks Uncle Newt questions about her parentage. She had put off asking the questions before now, even though she had her doubts for a while.
Uncle Newt is able to tell her that Susanna Noone was indeed her mother, despite Vanessa’s resemblance to Aunt Gally.
He does go on to say that Gally would have loved to have been Vanessa’s mother, and that, really, she was one of her two mothers in every meaningful sense.
When Vanessa asks what her father looked liked, Newt only says that he never met Major Noone.