Believing in creationism is a sin

[Godscreated]

You are correct to the point of bothers and sisters marrying and having children, at the time it was not considered incest, the gene pool was nearly perfect and began to be degraded with time. This was due to sin and it's continued effect on mankind.

What is it that you think incest means? It has nothing to do with the purity of the gene pool; definitionally, if a brother marries his sister, that's incestuous. I understand your need to retreat from that word in relation to your creation story, but perfect genes or not (incidentally there's no mention of genes in the bible, and hence you're not really pulling that idea from the official source, there. You're just making up an excuse that feels better.) it's still an incestuous story.

Or are you saying that if a brother and sister wanted to get married today you wouldn't consider it incest so long as their genes were perfect?

Incest is a brother and sister marrying and or having sex. As for genes to be mentioned in scriptures, do you really believe those of that time would have known what God was talking about, God knew, He designed man. What the people would recognize is the unhealthy offspring of such a union. Just because I use today's knowledge does not mean it can not be applied to scripture, what we have and are learning was known to God even before creation.
I'm not making up anything, God knew about genes and how long it would take for them to become corrupted to a point that brothers and sisters having babies would result in children with birth defects leading to an unhealthy population. Man when Christians use their brains they are criticized without cause, and when they don't they are criticized and rightfully so. God put an end to brother sister marriage when he gave the law to Moses.
There is no perfect gene pool, so no a brother and sister should never marry.