I know, it stems from interbreeding and the problems associated with it. But, is that truly a hindrance with today's scientific advancements? Let's forget the so-called religious bans as the bible clearly hints that incest had to occur in there were so many offspring so quickly.
Read the article @
Lawyer Legalize incest Home Life Links DW.COM 17.08.2015
There is always someone who doesn't agree with incest laws.
Really the inbreeding argument is no longer valid- for lots of reasons.
However, I think that the argument for opposing first degree incest- siblings/parents/grandparents- really has to do with the very real potential for abuse- an older sibling or parent can far too easily intimidate or have undue influence- much the same reason as the prohibitions against sexual relations with your doctor or minister.