Thank you for the question on the descendants of Adam and Eve, “How did we multiply on Earth, since it was only Adam and Eve that were first created and it’s forbidden for a brother to marry among his sisters?”
As one reads Genesis 1–2, God didn’t prohibit the marrying of brothers to a sister. The command by God is for Adam and Eve with their descendants to populate the earth.
The prohibition of marrying a close relative comes much later through the Law given to Moses to the tribes of Israel.
SUMMARY: The Mosaic Law was not given to Adam and Eve.