Albert Barnes Commentary Judges 15:2

"And her father said, I verily thought that thou hadst utterly hated her; therefore I gave her to thy companion: is not her younger sister fairer than she? take her, I pray thee, instead of her." — Judges 15:2 (ASV)

I gave her - In marriage. Samson had probably not heard of this before. Samson’s father had paid the dowry for the older sister; her father therefore offers her sister in her place. The fear of Samson probably also influenced him.