Rashi on Genesis
בת בתואל DAUGHTER OF BETHUEL — She answered his first question first and his last last (cf. Avot 5:9).
Or HaChaim on Genesis
בת בתואל..אשר ילדה לנחור. The daughter of Bethuel whom Milkah bore for Nachor. Rebeccah emphasised that she was not the daughter or granddaughter of a concubine, but of the ranking wife.
Radak on Genesis
ותאמר , she answered him on his first question; seeing that Milkah and Betuel were well known she told him all this.