If sin is the violation of commandments, how did people before Moses commit sin as they did not have the commandments

One is judged against his conscience, and not against any commandments. But one is obliged to form his conscience according to the moral law. Divine revelation was progressive in nature. The ten commandments were given at some point of time in the revelation history. Ever since our understanding of the moral law was evolving. It’s not that the moral law was evolving; our understanding of the revelation is growing up. 

People before the era of ten commandments were obliged to form their conscience based on general revelation and any of the special revelation given till their time. For example, polygamy was the norm in olden days. Only in the later part of the revelation history we could know the truth. Hence the forefathers who were practising polygamy cannot be judged according to the present world moral standards. 

In any case, only God knows what is truly in the hearts of men and only God judges their virtue.