I am wondering how all the nations came to be. If there were only two people in the beginning, how did they multiply and become different nationalities? The only way I see is through incest. Incest is a sin so I just don't get how the world is filled with people but started with only two. It doesn't make any sense.
You are correct that incest as we know it today is a sin. However in the beginning inbreeding wasn't. Your thought process is also correct of how the human race began. Intermarriage occurred back in the original days. God ordained this to be.
Now Amram took for himself Jochebed, his father’s sister, as wife; and she bore him Aaron and Moses. And the years of the life of Amram were one hundred and thirty-seven. Ex. 6:20
This was before the law prohibiting it came to be as stated in the book of Leviticus which reads;
"None of you shall approach to any that is near of kin to him...." Lev. 18:6
In ancient times family members did marry. It was common for cousins to marry for example. In the book of Genesis, it gives some of the original genealogy from Adam and Eve. Chapter four gives the descendants of Cain (Adam's son), then in chapter five it continues with the genealogy of Adam and his descendants. (which do not include Cain or Abel). This genealogy spanned hundreds of years, and eventually led to Noah and Abraham. It was not considered incest until God gave the commandment to not continue with this method of reproduction. Our genetic code was much different back then as it is today. Today, reproduction within family members results in serious health issues that were not an issue originally.