‘And Laban said, “It is not so done in our place to give the younger before the firstborn. Fulfil the week of this one and we will give you the other also for the service which you will serve with me, yet seven more years.”
The taking of a second wife is well witnessed elsewhere, as is the later taking of slave-wives. But for the main wives there would be legal stipulations in the marriage contract, either written or oral and made in the presence of witnesses, preserving their position and relative freedom. The marrying by one man of two sisters was, however, later forbidden (Leviticus 18:18).
Laban knew that Jacob would have to recognise the strength of his argument. Custom could not be broken. Every one in the tribe would know the situation, and they were no doubt smiling behind Jacob's back. And behind his triumphant but partly concealed smile is the implication that Jacob should have known, and that had he been smarter he would have known. It was probably not an uncommon requirement, although marriage to the elder daughter did in fact place Jacob in a more privileged position. (An argument which Laban might well have called on when placating Jacob. Marriage was not on the whole looked on as a romantic affair).
However Laban is not averse to Jacob and placates him with a further offer. Let him go through the seven-day wedding feast (see Judges 14:12) without trouble, giving Leah her full due, and then he can also marry Rachel. After which he must work another seven years for the privilege, as a now privileged member of the tribe.
It has been suggested in the light of parallels elsewhere that Laban adopts Jacob as a son, but there is nothing in the narrative to suggest this and much to demonstrate that he retained a level of independence. He was an established member of the family tribe, connected by marriage, but his services had to be retained by contract. Thus the new seven year contract.