Reuben and Bilhah
Reuben was the firstborn son of Jacob. His mother was unloved-by-Jacob Leah, who was given in marriage by means of a deception (see Jacob's First Love):
"29:31 And when the LORD saw that Leah was hated, he opened her womb: but Rachel was barren.
29:32 And Leah conceived, and bare a son, and she called his name Reuben: for she said, Surely the LORD hath looked upon my affliction; now therefore my husband will love me." (Genesis 29:31-32 KJV)
Bilhah was Rachel's maidservant. When Rachel found herself not having children as she wanted, she appointed Bilhah as a surrogate wife and mother. Bilhah's concubine marriage produced two children with Jacob i.e. Bilhah was the concubine-wife of Reuben's father Jacob, and the mother of two of Reuben's brothers.
"30:3 And she said, Behold my maid Bilhah, go in unto her; and she shall bear upon my knees, that I may also have children by her.
30:4 And she gave him Bilhah her handmaid to wife: and Jacob went in unto her.
30:5 And Bilhah conceived, and bare Jacob a son. 30:6 And Rachel said, God hath judged me, and hath also heard my voice, and hath given me a son: therefore called she his name Dan.
30:7 And Bilhah Rachel's maid conceived again, and bare Jacob a second son. 30:8 And Rachel said, With great wrestlings have I wrestled with my sister, and I have prevailed: and she called his name Naphtali." (Genesis 30:3-8 KJV)
"Unstable as water, thou shalt not excel; because thou wentest up to thy father's bed; then defiledst thou it"
With Reuben being the eldest son of Jacob, and Bilhah being a young maidservant (and mother of two small children), there was likely not much age difference between them. But their age was irrelevant to the adultery ("extramarital sex") and (perhaps, technically) incest that they committed.
"35:21 And Israel journeyed, and spread his tent beyond the tower of Edar.
35:22 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.
Now the sons of Jacob were twelve:
35:23 The sons of Leah; Reuben, Jacob's firstborn, and Simeon, and Levi, and Judah, and Issachar, and Zebulun:
35:24 The sons of Rachel; Joseph, and Benjamin:
35:25 And the sons of Bilhah, Rachel's handmaid; Dan, and Naphtali:
35:26 And the sons of Zilpah, Leah's handmaid; Gad, and Asher:
These are the sons of Jacob, which were born to him in Padanaram." (Genesis 35:21-26 KJV)
It was a very costly mistake for Reuben. Years later, in Egypt, when Jacob was giving the prophetic blessings to his sons, Reuben lost his place as the firstborn, to Joseph (ironic, since it was Reuben that saved Joseph from being killed when he was sold into Egypt - see the Fact Finder question below), specifically because of the incident with Bilhah.
"49:1 And Jacob called unto his sons, and said, Gather yourselves together, that I may tell you that which shall befall you in the last days. 49:2 Gather yourselves together, and hear, ye sons of Jacob; and hearken unto Israel your father [see Jacob's Family].
49:3 Reuben, thou art my firstborn, my might, and the beginning of my strength, the excellency of dignity, and the excellency of power: 49:4 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.
49:5 Simeon and Levi [see Levites] are brethren; instruments of cruelty are in their habitations. 49:6 O my soul, come not thou into their secret; unto their assembly, mine honour, be not thou united: for in their anger they slew a man, and in their selfwill they digged down a wall. 49:7 Cursed be their anger, for it was fierce; and their wrath [see Dinah and Shechem], for it was cruel: I will divide them in Jacob, and scatter them in Israel.
49:8 Judah, thou art he whom thy brethren shall praise: thy hand shall be in the neck of thine enemies; thy father's children shall bow down before thee. 49:9 Judah is a lion's whelp: from the prey, my son, thou art gone up: he stooped down, he couched as a lion, and as an old lion; who shall rouse him up? 49:10 The sceptre shall not depart from Judah, nor a lawgiver from between his feet, until Shiloh come; and unto him shall the gathering of the people be [see Zionism]. 49:11 Binding his foal unto the vine, and his ass's colt unto the choice vine; he washed his garments in wine, and his clothes in the blood of grapes: 49:12 His eyes shall be red with wine, and his teeth white with milk.
49:13 Zebulun shall dwell at the haven of the sea; and he shall be for an haven of ships; and his border shall be unto Zidon.
49:14 Issachar is a strong ass couching down between two burdens: 49:15 And he saw that rest was good, and the land that it was pleasant; and bowed his shoulder to bear, and became a servant unto tribute.
49:16 Dan shall judge his people, as one of the tribes of Israel. 49:17 Dan shall be a serpent by the way, an adder in the path, that biteth the horse heels, so that his rider shall fall backward. 49:18 I have waited for thy salvation, O LORD.
49:19 Gad, a troop shall overcome him: but he shall overcome at the last.
49:20 Out of Asher his bread shall be fat, and he shall yield royal dainties.
49:21 Naphtali is a hind let loose: he giveth goodly words.
49:22 Joseph [see also Ephraim and Manasseh; also Why Did Jacob Adopt Ephraim And Manasseh?] is a fruitful bough, even a fruitful bough by a well; whose branches run over the wall: 49:23 The archers have sorely grieved him, and shot at him, and hated him: 49:24 But his bow abode in strength, and the arms of his hands were made strong by the hands of the mighty God of Jacob; from thence is the shepherd, the stone of Israel: 49:25 Even by the God of thy father, who shall help thee; and by the Almighty, who shall bless thee with blessings of heaven above, blessings of the deep that lieth under, blessings of the breasts, and of the womb: 49:26 The blessings of thy father have prevailed above the blessings of my progenitors unto the utmost bound of the everlasting hills: they shall be on the head of Joseph, and on the crown of the head of him that was separate from his brethren.
49:27 Benjamin shall ravin as a wolf: in the morning he shall devour the prey, and at night he shall divide the spoil.
49:28 All these are the twelve tribes of Israel: and this is it that their father spake unto them, and blessed them; every one according to his blessing he blessed them." (Genesis 49:1-28 KJV)
Fact Finder: Along with the other sons of Jacob, what was Reuben's involvement in the selling of his brother Joseph into slavery in Egypt? Was it Reuben that kept the others from killing Joseph?