Three is an Orgy by Lorain Cicero

From the back:

Before she could object, he was under the covers beside her, his arms holding her tightly against him. She liked the sensation. After awhile, she stopped shaking from the cold, and a warmth began to seep through her.

'Thanks,' she said.

'My pleasure,' he said, and when she laughed softly he gave her a quizzical look.

'I was just thinking... you told me you were the runt of the family, but -' Her hands reached out for him, boldly.