1 So Isaac called for Jacob and blessed him. Then he commanded him: “Do not marry a Canaanite woman. 2 Go at once to Paddan Aram, to the house of your mother’s father Bethuel. Take a wife for yourself there, from among the daughters of Laban, your mother’s brother.
Isaac commanded Jacob to find a daughter of Laban to marry.
10 And it came to pass, when Jacob saw Rachel, the daughter of Laban, his mother's brother and the sheep of Laban, his mother's brother, that Jacob drew near and rolled the rock off the mouth of the well, and he watered the sheep of Laban, his mother's brother.
Now Jacob had found one. The expression "Laban, his mother's brother" is repeated 3 times in this 1 verse. This emphasis on long-lost relatives continues:
12And Jacob told Rachel that he was her father’s kinsman, and that he was Rebekah’s son, and she ran and told her father.
13 As soon as Laban heard the news about Jacob, his sister’s son, he ran to meet him and embraced him and kissed him and brought him to his house. Jacob told Laban all these things, 14and Laban said to him, “Surely you are my bone and my flesh!” And he stayed with him a month.
In this passage, there is an emphasis, even an over-emphasis of long-distance relatives being united. Modern people have lost some of this sympathy towards ones' relatives.
Why in Genesis, it's strongly said that Jacob and Rachel were Cousins?
First, it emphasizes the fulfillment of the wish of Isaac. Second, it depicts the joyful feelings of meeting long-distance relatives for the first time. Third, it demonstrates that blood is thicker than water.
All these are in contrast to the sorry affair of Esau marrying unrelated Canaanite women.