Most of Genesis 35 concerns movement first toward Beth-el and the sacrifice Jacob planned there, and then on toward Mamre where Jacob’s aged father Isaac lived. Then amid these various details, as it were out of the blue, we are told, “And it came to pass, when Israel dwelt in that land, that Reuben went and lay with Bilhah his father's concubine: and Israel heard it.” (Gen 35:22) And that is all that is said here about the incident. Is there some reason why this report is placed here and not, say, in Gen 38, where another instance of incest is recorded?
Laying with a man’s concubines could have been not merely an act of lust but also of political ambition. To lay claim to his concubines was practically tantamount to laying claim to the rest of the man’s possessions. That, at least, seems to be the upshot of the incident in which the wicked prince Absalom lay with the concubines of his father, the king, David (2 Sam 16:15-23). Allen P. Ross in his Bible Knowledge Commentary adds, interestingly: “It is possible that Reuben, Jacob’s eldest, was trying to replace his father as patriarch prematurely by this pagan procedure.”
That the act had a grasping, ambitious meaning is by no means certain, but seems probable, insofar as “Israel heard it” (Gen 35:22), i.e., took notice of it, and this notice had significant consequences for Reuben’s inheritance. On his deathbed, Jacob declares to Reuben, “Unstable as water, thou shalt not excel; because thou wentest up to thy father's bed; then defiledst thou it: he went up to my couch.” (49:4)
This then gives us a reason why the text is placed here. Jacob’s first three sons, Reuben, Simeon, and Levi, have turned out poorly (the latter two for the murder recounted in Gen 34) and are passed over in his father’s favor, leaving Judah, the fourth son listed in Gen 29, as one “whom thy brethren shall praise” (49:8). Similarly, Judah will be favored as the sire of the Messianic “lawgiver” unto whom “shall the gathering of the people be” (49:10). Already, then, we see a sifting among the sons of Jacob, occasioned by their own serious sins. As we see later in the text, what emerges is a question whether the primary heir of promise will be found in the line of Judah (the eldest son of Leah who had not caused himself to be disinherited) or of Joseph (the eldest son of Rachel). Of course, the true "seed of Abraham" does eventually emerge from Judah.