yes its weird if you don't understand and confused.
Try to broadened your mind and look for the right answer.
There is a span of time we do not know where the possibility of cain having sisters are not mentioned but that's not the point the writer of genesis wanted to figure out.
Gen 4:2 And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
Gen 4:3
And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.
Gen 4:16 ¶ And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden.
Gen 4:17 And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch.
In Genesis 5:4
we read a statement that sums up the life of Adam and Eve: “After he begot Seth, the days of Adam were eight hundred years;
and he had sons and daughters.”
During their lives, Adam and Eve had a number of male and female children. In fact, the Jewish historian Josephus wrote, “The number of Adam’s children, as says the old tradition, was
thirty-three sons and twenty-three daughters.”
Scripture doesn’t tell us how many children were born to Adam and Eve, but considering their long life spans (Adam lived for 930 years—Genesis 5:5), it would seem logical to suggest there were many. Remember, they were commanded to “be fruitful, and multiply” (Genesis 1:2

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