How could Jacob not have known that he took the wrong sister to bed (in Genesis)?

The story you're referring to is found in Genesis 29:15-30, where Jacob agrees to work for Laban, his uncle, for seven years in exchange for marrying Laban's daughter Rachel. However, Laban deceives Jacob by giving him his older daughter Leah instead of Rachel on their wedding night.

There are a few factors to consider regarding Jacob's apparent lack of awareness:

Cultural Practices: In the biblical context, it's important to understand the cultural practices of the time. In ancient Near Eastern cultures, particularly in the context of arranged marriages, it was customary for the bride to be veiled during the wedding ceremony. This could have made it difficult for Jacob to discern Leah's identity, especially in the darkness of night.

Deception by Laban: Laban was complicit in the deception by orchestrating the switch of brides. Laban's actions were motivated by his desire to marry off his older daughter Leah before his younger daughter Rachel. Laban's deception would have further complicated Jacob's ability to recognize the difference between Leah and Rachel.

Possible Consummation the Next Morning: In some interpretations of the biblical text, it's suggested that it was only the next morning, after Jacob had consummated the marriage with Leah, that he realized Laban had deceived him. By this time, it would have been too late to rectify the situation, as the marriage would have been considered legally binding.

Supernatural Explanation: Some interpretations also suggest a divine or providential aspect to the story, implying that God allowed the deception to occur as part of His larger plan for Jacob's life and lineage.

Overall, while it might seem implausible to modern readers that Jacob could have been deceived in such a manner, understanding the cultural and narrative context of the biblical story provides a framework for interpreting Jacob's actions and the events that transpired.