Adam Clarke on Genesis 29:26

It must not be so done in our country - It was an early custom to give daughters in marriage according to their seniority; and it is worthy of remark that the oldest people now existing, next to the Jews, I mean the Hindoos, have this not merely as a custom, but as a positive law; and they deem it criminal to give a younger daughter in marriage while an elder daughter remains unmarried. Among them it is a high offense, equal to adultery, "for a man to marry while his elder brother remains unmarried, or for a man to give his daughter to such a person, or to give his youngest daughter in marriage while the eldest sister remains unmarried." - Code of Gentoo Laws, chap. xv., sec. 1, p. 204. This was a custom at Mesopotamia; but Laban took care to conceal it from Jacob till after he had given him Leah. The words of Laban are literally what a Hindoo would say on such a subject.