Jamieson, Fausset and Brown on Genesis 34:5

5. Jacob held his peace--Jacob, as a father and a good man, must have been deeply distressed. But he could do little. In the case of a family by different wives, it is not the father, but the full brothers, on whom the protection of the daughters devolves--they are the guardians of a sister's welfare and the avengers of her wrongs. It was for this reason that Simeon and Levi, the two brothers of Dinah by Leah [Ge 34:25], appear the chief actors in this episode; and though the two fathers would have probably brought about an amicable arrangement of the affair, the hasty arrival of these enraged brothers introduced a new element into the negotiations.