I guess what I’m trying to point out is that moral laws are not arbitrarily decreed by God just because He wants them to be that way. Murder isn’t immoral because God wants it to be so. In other words, murder doesn’t offend God because it breaks some law He established…it offends God in and of itself, through the nature of what it is. God then gives us the law because it offends Him, and because He wants us to know that it offends Him.
It’s a slightly different situation from this one, however, since in this case God is declaring a law for mankind, not because it is an action that inherently offends him, but simply because it is for our own good. Intentionally breaking this law would be a sin of disobedience against God, while the previous example would additionally (and perhaps more deeply) offend Him by the very nature of what it is. It’s just a subtle distinction that I think is important to keep in mind here. God couldn’t turn around and declare tomorrow that murder is morally acceptable…but He theoretically could (and obviously did in Genesis) allow for brothers and sisters to be married.