She distinctively remembers Rickon's birth. Mother screamed so loud, it was said it could be heard in all of Winterfell, and Father left the birth chambers with decreased hearing for days afterwards.
But here's the key detail: Father was there. For that memory, Sansa had thought husbands are always present at their children's births. But no, it's not the case for everyone. Father's (former?) best friend leaves the castle when Margaery goes into labor—and she overhears someone saying he did the same when Cersei's bastards were born.
(She wonders—was the Kingslayer there, holding his sister-lover's hand? Of course, she'd never ask such a question, even if she wasn't being held hostage.)
As Sansa is no longer the queen's lady-in-waiting, she only hears of the birth hours after it happens, when Ser Loras comes to her saying his sister wants to see her. He frowns when saying that, as if he can't understand why. He doesn't seem to know that Margaery aided Arya's escape—an event that put her and Father in trouble, but quickly attributed to her sister's small size and adventurous nature.
As Ser Loras opens the door, she catches sight of the queen lying on a clean bed, her head resting on three pillows. The knight closes the door, leaving the two alone, and Sansa notices she is feeding her baby. "Come see her", Margaery calls, smiling softly.
Reluctantly, she obeys, sitting down on a chair next to the queen. "Where are your ladies?", she asks.
"They've come and gone", she replies. "I wanted you to see her too, and since I can't have you near them anymore…" The baby burps, and she chuckles as she takes her off her breast. "This is Arwyn Baratheon, heiress to the Iron Throne. Come, hold her."
There is no hesitance this time; Sansa has practice in holding babies, thanks to Bran and Rickon. She carefully takes Arwyn from her mother's grasp and raises her head to her eye level. The newborn princess has dark hair and hazel eyes, just like her mother. "She's beautiful", Sansa whispers.
"Indeed", the queen agrees. "Thank you for all your support, Lady Sansa." She looks at Margaery, startled, but she doesn't have time to counter that. "You've given me more tips than any other lady in this castle, even my grandmother, who seems to have forgotten how it was to be pregnant. If it wasn't for you, I'd have been tempted to ask my husband to bring his former wife back to assist me." She chuckles at her own words. "I know you were sharing your mother's experience, not yours, but it was much helpful. And you've been holding yourself spectacularly well, given your troubles. I will do all I can to relieve you and your father from this."
Sansa nods, handing little Arwyn back. She is unsure of how Margaery intends to fulfill her promises. As expected, Jon hasn't shown up or given any signs that he's alive, and neither has Mother. There was a letter from Maester Luwin claiming she was unwell and unable to travel, which has calmed the King now for a while, but she can see he's growing agitated again. "I am already grateful for your help, Your Grace", she whispers back. "For what you've done to Arya, and the trust you've placed upon my family."
Margaery grimaces. "Only a fool would deem the Avatar a threat", she states as a way of answering her gratitude. "Even if he was the Kingslayer. And I may not be able to do much, but I won't back up my husband's thirst for Targaryen blood. Now that I'm… suitable for my marital duties again, perhaps I can use it in your favor."
Sansa gasps. "No, Your Grace", she all but begs. "We are not worth this—"
"Your kin's lives are at stake, Lady Sansa, as well as yours. It is a small price to pay, and I won't hear you protest over it again."
That effectively shuts her up, and she only nods in gratitude before leaving.
A sennight after Princess Arwyn's birth, Ser Loras goes to her door. "Get ready for the feast tonight in the princess' honor", he tells her mechanically. Then he extends a hairnet for her. "And wear this", he instructs. "It's a gift from House Tyrell."
She grabs it and thanks him before closing her door and examining the hairnet. It's decorated with black gems, unlike anything she'd expect from the Tyrells—and who does Ser Loras mean by 'House Tyrell'? Queen Margaery? Her grandmother? One of the queen's cousins/handmaidens? Ser Loras himself?
Regardless, she knows she must wear it tonight, so she picks a grey dress to go with it. As always, she icebends small flakes on her braided hair, to remind everyone she is a Stark—no matter how bad her House's reputation is now.
She meets her father on their way to the hall, and nobody stops her from hugging him tight. "I missed you so much, Father", she nearly cries.
He hugs her back. "I missed you too, Sansa", he whispers.
They walk together to the hall and sit side by side. No one really stops them.
The Queen passes by their table to present her daughter to them, with her cousin Elinor by her side. Her grandmother passes by to tell her of her betrothal to Luthor Tyrell. "It will all depend on whether the king will release you or imprison", she admits. "I do hope it's the former case."
Ser Loras stands near them, watching them over, most of the time, except for when he switches with Ser Mandon Moore.
The king does not bother coming to them.
She makes small talk with her father as they dine and watch the crowd. She wishes they could hold real, meaningful conversations, but all of the possible subjects are forbidden here, so she enjoys her father in whatever way she can.
The king proposes a toast for his daughter and heir, and they all cheer. Not long afterwards, though, he begins to choke.
Her father stands up and tries to rush to his (former) friend's aid, but Ser Mandon stops him. The two of them watch helplessly as King Robert falls to the floor, gasping. Too many people surround him for them to see what is happening, but they can hear the gasps growing more and more desperate until they stop.
"The king is dead!", someone shouts.
Chaos ensues, and Ser Mandon holds both of them tight. Neither she or Father try to resist, fully aware of the implications of their actions, but Ser Loras marches to them with furious eyes anyway. "The king was poisoned", he snarls. "You are among the main suspects. Ser Mandon, lead Lord Stark to the dungeons. I'll scort Lady Sansa back to her chambers."
In the end, her hairnet incriminates her. When she argues that it was Ser Loras who gave it to her, claiming it to be a gift from his House, no one believes her, and she goes further in trouble by 'trying to place blame on the Queen's family'.
Renly Baratheon, who is the new Regent now that baby Arwyn is Queen, orders her to be sent to the Silent Sisters in Duskendale, while her father is to await judgement in the cells—for, if she did the deed, he must be the one who ordered it.
Her last visitor is Lord Varys, coming from the secret passages. "I cannot sneak you out this time", he laments, "but I can pass a message to you. You won't like it, though."
Her heart sinks. "What is it?"
"Lady Arya was never spotted in White Harbor. Whatever happened, her ship did not make it there."
She's crying when Ser Barristan comes to fetch her and take her to the ship to Duskendale. They all assume it's due to her punishment, but what is a life with the Silent Sisters in comparison to the news that her sister has no life at all to live anymore?