Andromeda Black cannot keep a secret to save her life. So when she becomes quiet and secretive, her sisters immediately sense that something is up. They do not pester her or ask her or even let her know that they know. They simply let her be.
They wait for almost a full year before the subject is raised. On their way home after getting off the Hogwarts Express, they start on their interrogation. (Sisters don't like secrets, really. It's selfish to keep something to yourself. They've shared everything for years so why should they start now?)
"Spill it," Bellatrix Black order crisply. She is the eldest so she has the first say in the matter.
"Please, Andy," Narcissa Black says gently. She is the youngest, the baby. Even Andromeda cannot say no to her.
"What am I supposed to say?" Andomeda Black, the middle child, asks in bewilderment. She wonders what brought this on. But she knows that they know. You can't live with your sisters for all of your life and hope to keep something from them. "I've never kept anything from you."
"Something has happened at school," Bella says quietly. Her dark eyes are fiercely protective and something stabs at her heart. They were the closest. Bella will never forgive her... "You're spending an awful lot of time with that Mudblood, Tonks. Don't think I didn't notice, Andy."
A blush creeps up her neck but she shakes her head. She is the obedient daughter, the kind sister. Of all the people in the world, she is the least possible person to betray the Black family. "It's nothing, Bella. We used to be partners in class. I never liked him."
Bella and Cissy looks at her as though they can see through her lies but they let it slide.
Andy smiles faintly. "I've never lied to you, have I?"
"You never said anything either," Cissy says softly.
Bella glances at her and smiles. It is not the beautiful and innocent Bella that she sees but the dark and dangerous Bella with the bared teeth. "You've got two choices, dear sister. You can have either your family or your lover. But let me remind you that you can never have both."
Andromeda shivers and looks away. If she has a secret, she must guard it with her life. Mustering a small smile, she says, "I'm a Black, aren't I?"