This was one of the things that Loki had mentioned would happen. He had expressly told Billy that Wanda would try to claim that he was one of her sons, who had died years ago. So when she got done speaking, he wasn't surprised. In fact, he was rather neutral about the whole thing. As she spoke though, he had pulled one of the rune stones that Loki had given him out of his pocket, rubbing it with his thumb like a worry stone.
"Wanda," he began, slowly and quietly. "I know you grieve for your children that were taken... but I'm not one of them." Regardless of what Loki had said about her, her wickedness and deviousness, he felt sympathy for her, even if it was simply an act.
"I have a mother, and a father. Besides, unless your twins were born over twenty years ago, how could I be one of them? I was already alive when these events occurred. Maybe your children live on, their souls reborn. But I don't think I'm one of them. I don't see how that could be possible."
He only hoped his words would defuse the situation, not accelerate it. At the moment, he kind of wished he had some backup after all, just in case.