(BTW, @Richca: even if they claim “Eve gave birth to a daughter who isn’t recorded in Scripture”, they still have to explain why brother-sister incest is a good thing (since we know that it runs counter to God’s Natural Moral Law.)
If brother-sister marriage were not a good thing in the beginnings of the human race, then you and I wouldn’t be here today nor anybody else.
Secondly, what woman can claim to be more akin to a man than Eve was to Adam, since of her did he say:
“This at last is bone of my bones
and flesh of my flesh;
she shall be called Woman,
because she was taken out of Man.”
Yet Eve was joined to Adam in marriage by God himself.
The fact is that the degrees within which consanquinity have been an impediment to marriage have varied according to the various historical times. We find among the Old Testament patriarchs and ancient Israelite history recorded therein marriages between persons that would not be allowed today according to the laws of the Church and some state/human laws. Abraham’s wife Sarah was his half-sister, his father’s daughter but not from the same mother. Jacob married his first cousins, his mother’s brother - Laban’s daughters.
Later, the Old Law given by God to Moses for the Israelites prohibited or permitted various degrees or lines of consanquinity involving lawful marriages. As St Thomas Aquinas remarks, the prohibitions appear to involve mostly all those persons who were wont to live together in one household in order to curb concupiscence or lust:
‘Wherefore as Rabbi Moses says (Doc. Perp. iii, 49) all those persons were debarred from marrying one another who are wont to live together in one household, because if a lawful carnal intercourse were possible between them, this would prove a very great incentive to lust. Yet the Old Law permitted other degrees of consanguinity, in fact to a certain extent it commanded them; to wit that each man should take a wife from his kindred, in order to avoid confusion of inheritances: because at that time the Divine worship was handed down as the inheritance of the race’ (ST, Suppl., Q. 54, art.4).
Upon the inaugeration of the New Law of Christ and his founding of the Church in which Christ instituted the sacrament of marriage, the Church has set which degrees of consanquinity are prohibited for lawful sacramental marriages and these degrees have varied according to the various times in the history of the Church.