"She is my only family, you know? I can't let anything happen to her."
The first time he saw one of those straw-heads in Bree, he knew that the Rohirrim would bring nothing but trouble. That final confirmation wasn't needed at all.
When he was told that his sister had been bedding that one specific straw-head, he almost freaked out. He knew that he wanted him dead, and he could have done something to the woman instead. He could have captured her, he could have tortured her. He could have killed her. Or, maybe more likely, he could have done nothing at all. But he was bound to know something: she was last seen with him.
He had to find out. It was a priority, the highest priority.
She was his only family, after all, and he couldn't let anything happen to her.
