The other “We” explanation is that the word used to identify God in the “E” tradition of the Torah/Pentateuch is a plural noun. The noun in the “J” tradition is singular.
This plural formation may be a leftover remnant from earlier tales in which the God of the Hebrews is the first among equals in a polytheistic religion that pre-dated the current monotheistic Judaism or it may be that assigning a plural name shows the greatness of God or there may be some other explanation. Anyone who stakes out a single “This is why” position will probably be challenged by others who hold different views.
The Book of Moses (found in the Pearl of Great Price makes it clear that Cain was born after Adam and Eve had other sons and daughters. (The Book of Moses is “An extract from the translation of the Bible as revealed to Joseph Smith the Prophet, June 1830-February 1831”).
To quote it:
Moses 5:2
“2 And Adam knew his wife, and she bare unto him sons and daughters, and they began to multiply and to replenish the earth.”
It isn’t until verse 16 that Cain and Abel are even born:
Moses 5:16-17
“16 And Adam and Eve, his wife, ceased not to call upon God. And Adam knew Eve his wife, and she conceived and bare Cain, and said: I have gotten a man from the Lord; wherefore he may not reject his words. But behold, Cain hearkened not, saying: Who is the Lord that I should know him?
17 And she again conceived and bare his brother Abel. . . .”
Cain and Abel weren’t Adam and Eve’s first children, according to the Book of Moses. Just my two cents, not trying to start a religious war or anything.