They were your brothers. You'd known them since they were born.
Theon/Reek: I deserved everything. I deserved to be Reek. I did terrible things. Turned on Robb. Captured Winterfell. Killed those boys.
Sansa: They weren't those boys, they were Bran and Rickon. They were your brothers. You'd known them since they were born.
Theon/Reek: They weren't, they were only...
Sansa: Only what?
Theon/Reek: I can't.
Sansa: Tell me.
Theon/Reek: I can't. Not unless the master says.
Sansa: Tell me. They weren't what? Tell me why Bran and Rickon should be gone, while you still breath the air. Tell me to my face Theon. Tell me that they weren't your brothers.
Theon/Reek: They weren't Bran and Rickon. I couldn't find them. It was two farm boys. I killed them and burned them so no one would know.