Later he found out she was his sister

This was 3 years ago. A friend he knew from his home town in Mexico asked him if a friend couple of him could spend a few days at his place in the US. A few days became 2 weeks and he decided they should start paying rent in some way. After 3 weeks they finally left his place and he never saw them again until 3 months ago when he went to Mexico to see his father. He found out he had a half-sister, daughter of his father and a neighbor. When he met her, he also found out she was the girl who almost 3 years ago tried to go to the US and stayed at his place.