"You mean Allison," Derek said, between bites. He didn't sigh, exactly, but there was some tension in his shoulders, the set of his mouth, before he answered.
"I suppose you deserve to know. It's not exactly a secret," he decided. "Scott and Allison and Stiles grew up in the human camps together. Scott and Allison were romantically involved, and then Scott was attacked and my sister, Laura, found him in the woods and turned him so he wouldn't die and invited him home with her. He refused and went back to the camps, but the other humans found out he'd been turned and rejected him, and he came to live with us."
He shrugged a shoulder and took a sip of lemonade, pausing before he continued. "I'm not sure how or when Stiles and Allison got captured, exactly, but it was probably only a few weeks later. I bought Stiles at the auction house where Scott found you, but another woman purchased Allison. Shortly after Stiles arrived, Scott became...troubled...and Laura sent him away to spend time with an ally of ours, Deaton. Scott may have mentioned him. It was Deaton's idea that Scott purchase a slave right away." The werewolf snorted, skeptically, opining on the sanity of such advice, and shook his head.
"And while Scott was away, Allison ran away from her Mistress and came to stay here. When Laura and I realized she'd been abused, we agreed she could stay here, as my slave, and then Scott came back, and I told Allison that she could decide whether to stay as my slave, or become Scott's," he finished, and took a large bite of his sandwich. Talking for long stretches wasn't his favorite activity, but he sensed Alcuin wasn't the type to interrupt, and it was better to get it all out, at once, and let him process it before asking clarifying questions.