Using your reasoning, because you cannot find any mention of the
incest law before Sinai, you gather that any kind of incest is not
a sin before Sinai.
I find this preposterous, because neither can we find other
commandments like "do not commit sexual immorality" or "do not
murder" before Sinai. Are you saying it is permissible to fark
around with other people's wife before the first mention of God
issuing an injunction against adultery?
Furthermore you missed the point in your reply about Cain. The
issue is not whether murder is a sin or not. It clearly is. It is
laughable that you dismiss any sinful act on the part of Lot's
daughter just because God did not warn them like He warned
Cain.
The topic is about incest. I'd say that incest was allowed to
happen until the time of Moses, for a very specific purpose that
GOD allowed to carry on their family line. This act was only
allowed at the early stages of history. Until Moses, i'd say that
effects of sin have caused the gene pool to break down and bring
about mutations, such that GOD puts a stop to it.
Is it morally wrong during the time of Lot? From our
perspective, yes because we live in the time after GOD says no
more. From the perspective of Lot's daughters then, it's not
morally wrong if it's to carry on the family line since maybe they
thought nobody else was around for them to reproduce with and
because of the effects of having recently come from Zoar.
But neither is it totally right, because there was enough
knowledge about good and evil then (effects of the fruit that Adam
ate?) for them to guess that this was morally unacceptable. Is it a
common thing? I don't know. Probably because other sources outside
of bible would talk about Nimrod and his mother. Does some sort of
law exist back then? Certainly! As Joseph declared to Potiphar in
Genesis, it is a sin for him to sleep with her.
Technically, was it a sin for Lot's daughters? No, because they
thought what they were doing was to repopulate the earth and not
for the pleasure, and as Origen said, it was a one off incident and
not meant to repeat again such that it becomes a consistent vice.
Is it a sin now? Definitely! Because GOD has specifically
disallowed it in Leviticus.