Sforno on Genesis 29:12/ספורנו על בראשית כ״ט:י״ב

1כי אחי אביה הוא, the Torah adds this at this point to justify his kissing Rachel and her allowing herself to be kissed by him.

2וכי בן רבקה הוא, he mentioned Rivkah to Rachel although Rachel did not know Rivkah at all. He did so because he wanted her to tell Lavan about who exactly Yaakov was.