Incest in Bible

I am always wondering how Bible can self justify about incest. Why we human had to start from one single couple and inevitably produced offspring by family incest? And then later on (thousands years later) God suddenly forbidden any incest relationship without any explanation? If someone say, we started from incest because God want us to be sinful from the begining. Why? Why can God make us in a better way? If he is so powerful and kind?